The Revision, Systematic Position, and Originof Diplodinium (Polyplastron) multi-vesiculatum and Diplodinium (Poly-plastron) bubali.

By

P. Wertheim, M.Sc, Ph.D.s

Zoological Institute, University of Yugoslavia, Zagreb.

With 7 Text-figures.

D i p l o d i n i u m (sub-genus P o l y p l a s t r o n ) m u l t i v e s i -c u l a t u m (Dogiel and Fedorowa, 1925) undoubtedly is one ofthe most interesting representatives of the Infusorian familyOphryoscolecidae (which occurs in the stomachs of ruminants),especially from the systematic and phylogenetic standpoint.The origin of this species shows some peculiar causes which areoperative in the development of these Infusorian species, butbesides that the present treatise points out some new aspectsregarding the development of new species in general.

First of all the systematic position of the species P o l y -p l a s t r o n m u l t i v e s i c u l a t u m must be cleared up, becauseowing to some recent investigations it has become obscure andconfused. This species was first discovered and described byDogiel and Fedorowa in 1925 under the name D. m u l t i -v e s i c u l a t u m . In 1927 Dogiel established for this speciesa new sub-genus P o l y p l a s t r o n . In 1928 Dogiel describedfrom the stomach of Buffe lus b u b a l u s L. another speciesof the same sub-genus, D. (Po lyp l a s t ron ) b u b a l i . Intheir revision of the genus D i p l o d i n i u m Schuberg, Kofoidand MacLennan have made P o l y p l a s t r o n an independentgenus and have divided it into three species. The first species isthe P . m u l t i v e s i c u l a t u m , the second P . f e n e s t r a t u m(Dogiel, 1927) (this form was described by Dogiel only as anaberration of the species P . m u l t i v e s i c u l a t u m ) , and thethird is P . m o n o s c u t u m (Dogiel's aberratio confluens) .

32 P. WERTHEIM

For the species from the stomach of B. b u b a 1 u s Kofoid andMacLennan have established a new genus with the single speciesE l y t r o p l a s t r o n b u b a l i . In 1933 Becker in a smalltreatise named the E. hegner i (Becker and Talbott, 1927)(with the following synonymy: D. hegner i Becker andTalbott, 1927, D . - P o l y p l a s t r o n - b u b a l i Dogiel, 1928,D . - P o l y p l a s t r o n - l o n g i t e r g u m Hsiung, 1931, andE. buba l i Dogiel, 1928; Kofoid and MacLennan, 1932),which already in 1927 he had been the f i rs t to describetogether with Talbott as a new species D. hegne r i , fromthe stomach of domestic cattle and sheep. But as under thisname both authors have described forms having a very hetero-geneous structure Kofoid and MacLennan identified some ofthese forms described by the former authors (under the nameD. hegneri) as O s t r a c o d i n i u m ob tusum (Dogiel andFedorowa, 1925). Becker agrees with them, but at the sametime points out that some of the forms described by himselfunder the name D. hegner i can in no case be included inthe group 0 . ob tusum being already identical with the formdescribed by Dogiel in 1928 as the new species D. (Poly-p las t ron) b u b a l i . Kofoid and MacLennan have discoveredthis form in the stomach of Bos i n d i c u s . In view of the lawof priority according to the international rules of zoologicalnomenclature and of what has been said above, the name givenby Becker and Talbott must alone be used although the latter'sdescription was less complete than that of Dogiel, and althoughthey have re-examined this form (Becker in 1938: 'The writerrecently re-examined some of the type material of D. he-gner i . . .') and only subsequently discovered (of course, onthe basis of Dogiel's description) some characteristics formerlyunnoticed by them and first described by Dogiel.

It is now most important to know whether Kofoid andMacLennan were right in raising P o l y p l a s t r o n from beinga sub-genus of the genus Dip lod in ium to being an inde-pendent genus. They also made independent genera of someother sub-genera, even of the species of the genus Diplo-d in ium. In my revision of the family Ophryoscolecidae(which I am doing now) I shall, of course, touch upon every

DIPLODINIUM 33

single case; while in this treatise I shall only put forward sucharguments as concern Polyplastron, making them, however,applicable in principle to all other cases. Eegarding the sevengenera of the family Ophryoscolecidae described up to thepresent day, Entodinium (Stein, 1858), Diplodinium(Schuberg, 1888), Epidinium (Crawley, 1924), Ophryo-scolex (Stein, 1858), Caloscolex (Dogiel, 1926), Opistho-trichum (Buisson, 1923), and Cunhaia (Hasselmann, 1918),there exist, fortunately, among them such differences that noone can any longer confuse them. Besides, it must be em-phasized that all representatives of the genera in question areas a rule so uniform and typical that it is easy to identify them.The chief generic difference is the s tructure ofthe ciliary or membranelle zone located in theanterior end of the body near to the mouth. Thiszone has a structure so uniform and characteristic in all repre-sentatives of the same genus that at the first glance throughthe microscope we may immediately determine the genus.Thus the genus Diplodinium is characterizedby two membranelle zones (adoral and dorsal),both of them in the same transverse plane,while between the two zones is found a well-developed processus apicalis, whereby the genusDiplodinium is undoubtedly distinguished fromother genera of this family (see Text-fig. 1). The genusDiplodinium was divided by Dogiel into four sub-genera (Anoplodinium, Eudiplodinium, Ostra-codinium, and Polyplastron), while Kofoid and Mac-Lennan have raised all these sub-genera into genera proper,adding also some new ones, so that according to them all thatformerly was included in the genus Diplodinium is nowseparated into the following distinct genera: Eodinium,Diplodinium, Eremoplastron, Eudiplodinium,Diploplastron, Metadinium, Polyplastron, Ely-troplastron, Enoploplastron, and Ostracodinium.They made this division by taking the more special character-istics (for example, the skeletal plates) as a criterion for thegeneric differentiation, which is a great logical error (divisio

NO. 309 D

34 P. WERTHBIM

sit a d a e q u a t a ! ) . In view of the fact that Ophryoscolecidaehave several genera, some with skeletal plates and some without,the skeletal plates cannot properly be taken as a typical criterionfor generic division (this would only be possible if the family hadonly two genera, of which one had skeletal plates and the othernot). That this is really so may clearly be seen in the case ofO p h r y o s c o l e x and E p i d i n i u m , because these two genera,so different and so sharply divided, have an identical structure

TEXT-FIG. 1.Adoral and dorsal membranelle zones of the Ophryoscolecidae.

X200. A, Entodinium sp.; B, Diplodinium sp.: a,Anoplodinium sp., 6, Eudiplodinium sp., c, Poly-plastron sp., d, Ostracodinium sp.; C, Epidinium sp.;D, Ophryoscolex sp.; E, Caloscolex sp.; F, Opistho-trichum sp.: 1, dorsal, 2, adoral membranelle zone.

of the skeletal plates. For this reason it is imperative to returnto Dogiel's original division, as Kofoid and MacLennan's newgenera, arrived at by the splitting up of the genus Diplo-dinium, are nevertheless more closely related to each otherthan to other genera of the family Ophryoscolecidae. Kofoidand MacLennan's division has two fundamental errors. Thefirst one is of a theoretical nature. According to their divisionwe make a mistake from the phylogenetic point of view,because all representat ives of the phylogeneti-callyuniform genus Diplodinium are being splitinto independent systematic categories taxo-nomically equivalent to other genera of the samefamily which are not so closely related to thisgroup, while on the contrary genera should be

DIPLODINIUM 35

e q u i v a l e n t u n i t s . The second error is of a practicalnature. Belated groups are being split, and an entirely un-necessary ballast of new names is created, causing taxonomicaland phylogenetical confusion. For this reason it is better toleave all the representatives of this group in one single genusD i p l o d i n i u m . In so far as there is a necessity for further

TEXT-FIG. 2. TEXT-FIG. 3.

P . m u l t i v e s i o u l a t u m . X300. P . hegne r i . X300.Dorso-ventral right view. Dorso-ventral right view.

differentiation, D i p l o d i n i u m may be divided into sub-genera. According to the arguments stated above, and basingourselves on the facts relative to the systematics of the familyOphryoscolecidae, we are actually forced to make a revision inthe sense that D i p l o d i n i u m sensu l a t i o r e b e re-erectedas a genus sensu l a t i o r e , with sub-genera. One of the sub-genera of the genus D i p l o d i n i u m is P o l y p l a s t r o n ,which interests us most here.

After this revision P o l y p l a s t r o n (see Text-figs. 2 and 3)has the following position:

Genus D i p l o d i n i u m Schuberg 1888.Sub-genus P o l y p l a s t r o n Dogiel 1927.Species P . m u l t i v e s i c u l a t u m Dogiel and Fedorowa,

1925.P . h e g n e r i Becker and Talbott 1927.

The following table shows the hosts of the two species as wellas their synonymy.

36 P. WERTHEIM

TABLE 1.

P. multivesiculatum and P. hegneri: Investigators,hosts, and synonymy.

Species.

1. P . mul t ives i cu la tum.Dogiel and Fedorowa,'Arch. f. Protistenk', vol.59, p. 130, figs. 73, 74,1927.

Synonymy:D. mul t ives icu la tum.

Dogiel and Fedorowa,'Zool. Anz.', vol. 62, p.100, fig. 4, 1925.

2. P . hegneri . Becker andTalbott, 1927.

Synonymy:D. hegneri . Becker and

Talbott (partim), 'IowaSt. Coll. Journ. Sci.', vol.4, p. 357, pi. 2, fig. 17,1927.

D. (Po lyp las t ron) bu-bal i. Dogiel,' Ann. Para-sitol.', vol. 6, p. 332, fig. 4,1928.

E. b u b a l i . Kofoid andMacLennan, 'Univ. Calif.Publ. Zool.', vol. 37, p.121, pi. 6, figs. 13,14,1932.

Host.

Bos taurus,Ovis aries,Caprahircus,0. orientalis

cycloceros,C. aegagrus.

B. taurus,0. aries,B. bubalus,B. indicus.

Investigator.

Dogiel and Fedorowa,Becker and Talbott,Wertheim.

Becker and Talbott,Dogiel, Kofoid andMacLennan.

Before proceeding farther, we must examine why the erectionof the species P. hegneri to the new genus Elytropla-stron is unjustified and why we are justified, both phylo-genetically and taxonomically, in including it in the sub-genusPolyplastron, which after all had already been done byDogiel. Similarly, we must prove the fallacy of Kofoid andMacLennan's procedure when making Dogiel's aberrations ofthe species P. multivesiculatum, i.e. P. multivesicu-latum aberratio confluens and P. multivesiculatumaberratio fenestratum, independent species. Dogiel, towhom we are indebted for a very complete description of

DIPLODINIUM 37

P. hegner i , points out that this species in general remindsone of the typical species P . m u l t i v e s i c u l a t u m (Dogiel,1928: ' . . . tres semblable au P . mu l t i ve s i cu l a tum) ,having, however, distinct features of a separate species ( ' . . . fontde P . buba l i une bonne espece'). Kofoid and MacLennan,too, when stating that 'although the paths of development ofthe two have been different', say, however, in regard to P .hegner i and P. m u l t i v e s i c u l a t u m : ' P o l y p l a s t r o nexhibits approximately the same grade of complexity asE l y t r o p l a s t r o n . ' P . hegner i and P . mu l t i ve s i cu -l a tum have the same general shape, the same structure of theecto- and endoplasm (which is very important!), the same formof the macro-nucleus and micro-nucleus and of the two rightskeletal plates; further they have the same shape and positionof the rectum and of the anus. The only difference is in thenumber of accessory contractile vacuoles and in the absence ofone of the left skeletal plates. These differences, however, aretoo small to justify the making of a separate genus. Kofoidand MacLennan themselves, for instance, have left the speciesMetad in ium medium and M. yps i lon in the samegenus Metad in ium, although the first of these two speciesshows the two right skeletal plates clearly separated, whilein M. yps i lon these two plates are fused (not to mentionother notable differences such as the general shape and size ofthe body, the shape and position of macro-nucleus and micro-nucleus), and again Kofoid and MacLennan say themselves(1932, p. 60): 'The skeletal plates are very constant, highlycharacteristic structures,' and further (1932, p. 118): 'In theother genera, the form of the plates is stable within a speciesand it seems unlikely that P o l y p l a s t r o n alone would bean exception.' It is evident that, if they had been consistent,they would, in view of the above-mentioned example, also haveto divide the genus Metad in ium into new and separategenera. If, in view of the great variability of all characteristicsincluding the skeletal plates of the family Ophryoscolecidae,their method were adopted, almost every second species wouldmake a separate genus. Furthermore, the same authors sayagain (1930, p. 497):' The most marked differences are presented

38 P. WEBTHEIM

by the macro-nucleus, both in position and shape, position ofthe contractile vacuole, the endoplasmic sac and rectum, andfinally the external characters such as shape, size,1 and thevarious projections in the form of lobes, spines, and flanges.No one character is sufficient to completely characterize aspecies and to establish its relationships within the genus. Thewhole complex of characters must be used.' Accordingly, bothauthors contradict themselves not only in the facts but also intheir own theses when in one case they put individuals withdifferent skeletal plates (besides other differences) into the samegenus (the case of Metad in ium) , while in another case theysplit, for the same reasons, two species into two separate genera(the case of P o l y p l a s t r o n and E l y t r o p l a s t r o n ) , whichthey should never have done, if they had perceived not onlythe morphological similarity but also the phylogenetic origin ofthe two species in question (Kofoid and MacLennan, 1932,p. 121: ' I t may be suggested that E l y t r o p l a s t r o n andP o l y p l a s t r o n have evolved from a form such as Dip 1 o-p l a s t r o n and in the same direction.') On the ground of theabove-stated arguments it is incorrect to divide these twospecies into two separate genera, a division into two clearlycharacterized different species of the same genus Dip lo -d i n i u m , sub-genus P o l y p l a s t r o n , being quite sufficient.

In addition to the typical P . m u l t i v e s i c u l a t u m ,Dogiel has described two more aberrations, i.e. conf luensand f e n e s t r a t u m , the first one with totally fused rightskeletal plates, the second with partly fused skeletal plates.Kofoid and MacLennan have erected these aberrations intoseparate species: P . m o n o s c u t u m (instead of using Dogiel'sname c o n f l u e n s , here Kofoid and MacLennan have quitecorrectly acted in conformity with the Art. 11 of internationalrules regarding zoological nomenclature, the name conf luenshaving already been occupied) and P . f e n e s t r a t u m .Dogiel has rightly named these forms ' aberrations', because heseldom found them, in great contrast with the very frequent

1 The identity of the characteristics enumerated here relating theP. multivesioulatum and P. hegneri having already been men-tioned before.

DIPLODINIUM 39

appearance of the typical species P . m u l t i v e s i c u l a t u m .The aberration conf luens Dogiel saw only once! In fact, itis impossible to erect a new species on a single aberrativeindividual! I had an opportunity myself of examining duringa year very rich populations of P . m u l t i v e s i c u l a t u m ofSomali sheep, having used every day fresh material from herstomach, and observed them both living and fixed (stained).During several months the P . m u l t i v e s i c u l a t u m , by thenumber of individuals, was the predominant member of this

TEXT-ITO. 4.

Transitional stages of the fusion of the right-side skeletal platesof P . m u l t i v e s i c u l a t u m . X300.

stomach infusorian fauna. The great number of individualsgave me a wide choice. By careful observation of the forms andthe position of the two right skeletal plates ( p r i m i t i v a andc a r i n a) in more than 700 individuals belonging to this species,I could see myself that in rare cases the gradual fusion of thesetwo skeletal plates was to be observed. Such fusion shows allpossible transitional stages (see Text-fig. 4).1 In view of the greatscarcity of such forms, we cannot speak in this case of anyseparate form, much less of a separate species; the more so asthey differ but slightly from the type, and as there exists aseries of gradual transitional stages which show that this formis a simple, individual, aberrative variation. Accordingly, theseso-called 'species' P . mono.scutum and P . f e n e s t r a t u mmust be discarded! Just these aberrant forms reveal to us thedirection in the evolution of the family Ophryoscolecidae andits great variability. Dogiel has shown (1925) how this variability

1 By using chlor-zinc-iodide or a combination iodine-alcohol and con-centrated HaSO4 one can get a nice reaction (brown to black) of the skeletalplates.

40 P. WBRTHEIM

is sometimes also conditioned and considerably increased bya special formation of some internal anatomical characters.

Now it is necessary to examine the position of the sub-genusP o l y p l a s t r o n within the genus D i p l o d i n i u m and tofind the line of evolution of the two very significant speciesP . m u l t i v e s i c u l a t u m and P . h e g n e r i . The complexstructure of the skeleton and the vacuolary apparatus of thesespecies must be deduced from one of simpler structure. If allthese characters are taken into consideration, there is only oneform D. (Eud ip lod in ium) aff ine (Dogiel and Fedorowa,1925) (synon.: D. aff ine Dogiel and Fedorowa, 1925, afterKofoid and MacLennan, 1932) which is the nearest to P o l y -p l a s t r o n . In this I fully agree with Kofoid and MacLennan,who (as cited above) very correctly noted the relation betweenthese species. From one single, narrow, skeletal plate of the samespecies of the Diplodinium—sub-genus E u d i p l o d i n i u m—leads the way to those species of E u d i p l o d i n i u m whichare equipped with two relatively narrow, right-side, skeletalplates, and these are E . aff ine and E . m e d i u m . However, inthe case D. (Eud ip lod in ium) m e d i u m t a u r i c u m thesetwo skeletal plates that extend from the posterior part fuse intoa single one (with transitional stages), leaving anteriorly a greateror smaller opening. We are, therefore, right in assuming thatthe wide, right-side, skeletal plate of the Diplodin ium—sub-genus O s t r a c o d i n i u m (Dogiel, 1927)—has evolved fromtwo narrower ones which had gradually fused into a single one.Dogiel, too, confirms this opinion. In this way E . aff ine wouldbe a progenitor of P o l y p l a s t r o n and E. m e d i u m of theO s t r a c o d i n i u m , as we see in the following scheme:

E. affine < > E. medium mediumI \ I

Polyplastron E. medium tauricum—E. ypsilon

P. hegneri P. multivesiculatum Ostraoodinium

P.m. aberr. fenestratum

P.m. aberr. monoacutumIt follows therefrom that P o l y p l a s t r o n ' s position

DIPLODINIUM 41

between E. affine and the descendants of E. mediummedium, being, however, itself distinguished from the latteras a separate group with orimental development of left-sideskeleton preserving, but slightly, their tendency for the fusionof p r im i t i va and ca r ina . Parallel with this growth of theskeletal plate goes the increase in number of contractile vacuoles,as will be shown in the following table:

TABLE 2.

Enlargement of the skeletal plate and increase in number ofcontractile vacuoles in the developmental line.

iP o l y p l a s t r o n

Os t r acod in ium

Species.

E. affineE. medium

mediumE. medium

tauricumP. hegneri

P. multives.

0. obtusumobtusum

Skeletal plates(right).

2 of middle width2 of middle width

are fusing

2 right of middlewidth

2 right of middlewidth1

1 large

Number ofcontractilevacuoles.

22

2

3 or 4

9

6

Notice.

If these were not a largeright-side skeletal plate,there would be no ob-stacle for a similardevelopment, in addi-tion to a number ofvacuoles along the dor-sal border, of another' irregular' vacuole,which can be assumedby the fact that withthe P . m u l t i v e s i -c u l a t u m aberr.monoscu tum the'irregular' vacuole No.7 disappears, because itwas ' driven out' by thefused skeletal plate.

According to the report of Kofoid and MacLennan and Tal-bott, P . hegner i was found by them also in domestic cattle;thereby extending our knowledge regarding its occurrence.The only host known before was B. b u b a l u s , according

1 With a tendency for fusion.

42 p. WERTHEIM

to Dogiel. According to their structure, P. multivesicu-latum and P. hegneri belong to the most complex descen-dants of the family Ophryoscolecidae. They belong to the mostevolved forms. From Eudiplodinium, having a constantnumber of two vacuoles, Polyplastron emancipates itselfwith a tendency to increase this number. However, it is stilla question why this number is increasing. My experimentalinvestigations of a number of vacuoles have led me to realizethe fact, which I could also ascertain numerically (for theP. multivesiculatum too), that the total surface and thesize of the contractile vacuoles stand in a definite relation tothe total surface and the size (Wertheim, 1934) of a certainspecies of the family Ophryoscolecidae. If we compare the sizeof the contractile vacuoles of P. multivesiculatum withthe size of those of D. (Eudiplodinium) medium, forexample, we see how small are those of P. multivesicu-latum, and their number ought therefore to be proportionallygreater in order to enable them duly to carry on their task. Wetherefore can establish the rule, that in the familyOphryoscolecidae the number of contracti levacuoles increases only parallel to the increaseof that given species, or as well by a simultaneousdecrease in the size of some existing vacuoles.This can be clearly observed in the case of the genus Ophryo-sco 1 ex as well as with the species 0 . obtusum (Dogiel andFedorowa, 1925). While studying the already mentionedabundant populations of P. multivesiculatum, I couldascertain that there were a few atavistic cases in which theforms P. multivesiculatum did not have the usual ninevacuoles but only two dorsal ones; but those were now muchlarger (in accordance with our above-mentioned rule; seeText-fig. 5.) These vacuoles, owing to their size, were influentialin changing the form of the macro-nucleus. This is an analogouscase—in a different direction—to that of Dogiel's (1925). Thecase of atavistic recurrence in these two vacuoles, typical forEudiplodinium, is another proof of the fact that Poly-plastron was derived from Eudiplodinium. Concerningthe skeletal plates, I could ascertain a gradual transition to the

DIPLODINIUM 43

E u d i p l o d i n i u m type with two right-side skeletal plates.Text-fig. 6 shows the orimental development of the left-side

TEXT-FIG. 5.

Contractile vacuoles of P. multivesiculatum. X400.

skeletal plates as I could ascertain them in the populations ofP . m u l t i v e s i c u l a t u m from the Somali sheep mentionedabove. Evidently, this enlargement of the skeletal apparatus

1 / t\TEXT-FIG. 6.

Oriments of left-side skeletal plates of P. multivesiculatum.X400.

means intensification of its function. Using chlor-zinc-iodide asa reaction for a stronger brown to black staining of the skeletalplates (reaction on the cellulose) I could observe that with theforms without skeletal plate a collapse would take place at agreat concentration of the reagent, while with the forms havingright-side skeletal plates it was somewhat slighter, as well aswith O s t r a c o d i n i u m , while E p i d i n i u m and 0 p h r y o -scolex were almost perfectly resistant. This means that the

44 P. WERTHEIM

latter are protected by the skeleton, which in this case envelopstheir body all around. Consequently, regarding the function ofthese skeletal plates, I can, on the basis of my observations,confirm the following fact: t h a t the ske le t a l p l a t e shave also the func t ion of s t r e n g t h e n i n g thebodies of these forms, not alone of the pharynx. Isuppose, owing to a very dense stomach-liquid, the body of theOphryoscolecidae must be resistant against lateral pressurefrom all sides. The cuticle, strengthened by a thick layer ofectoplasm (encrusted by silicic acid), has to serve this purpose,but in any case the skeletal plate may intensify this action. Itis characteristic that the relative thickness, of course, withregard to the size of the species in question, of the ectoplasmis less as the skeletal apparatus is more developed. To supportmy assertion I also mention the fact that the skeletal platesusually develop on the right and the left side of these Infusoria(not dorso-ventrally), but as their body is dorso-ventrallyflattened they are larger on the right and left sides. Where thesurface is larger there is also a greater pressure, so that conse-quently a greater resistance and hardening of the sides of thebody are necessary. According to this, cylindrical forms again(Epid in ium, Ophryoscolex) have not a pronouncedright-side skeletal apparatus but a cylindrical one, because whilein movement their more rounded body is more evenly exposedto pressure of the environmental fluids. The same applies toCaloscolex and O p i s t h o t r i c h u m . Os t r acod in iumis in this respect intermediate. If we consider these two repre-sentatives of the sub-genus P o l y p l a s t r o n , we shall observethat P . m u l t i v e s i c u l a t u m shows greater complexity thanE. a f f i n e (compare in Text-fig. 7 the identical parts in thebodies of P . m u l t i v e s i c u l a t u m , P . hegner i , and E.affine). In all probability the form P . hegner i is phylo-genetically older. As we can see from our comparative drawings,P . hegner i is closer to E. affine in the position of itsdorsal vacuoles, by the form of its macro-nucleus, and by thedirection of the right skeletal plates, than is P . m u l t i v e s i -cu l a tum to the E. aff ine. Thus a cue is given for thetendency of the development of left-side skeletal plates, as

DIPLODINIUM 45

shown in the present case of Diplodinium—sub-genusP o l y p l a s t r o n .

Finally the ways in the evolution of the P. mul t ives icu la -tum and P. hegner i must be explained, these two speciesbeing very interesting by the tendencies in their evolution, andas having a distinctive structure of their skeletons and theircontractile vacuoles. These species show how the evolution ofOphryoscolecidae opens up very many possibilities in a most

A, P . m u l t i v e s i c u l a t u m , B, P . hegner i , C, E u d i -p lod in ium affine (comparative drawings, dorso-ventralright view). P., processus apioalis; Ma., macro-nucleus; Mi.,micro-nucleus; Sk., skeletal plates; Ect., ectoplasm. A, x225;B, X300; 0, X750.

peculiar way. And the two species in question, which in thegenus Dip lodin ium have their own place, have just realizedone of those possibilities. They represent a new evolutionarytype which in the future can in this direction produce quiteindependent groups, the more so as I already proved (Wertheim,1934, Zool. Anzeiger) a 'caudopetal' tendency in P . mu l t i -ve s i cu l a tum. Speaking of the factors at work in theevolution of Ophryoscolecidae, it must first of all be pointedout that here the factor of selection in the form of some strugglefor life can in no case come into consideration, which is self-evident.

There is only one thing that must be kept in mind: great

46 P. WEETHEIM

masses of these forms perish daily with the passing of the foodinto the omasus and abomasus, where actually their remainsmay be found. As my former investigations have shown, thespecies of Infusoria are fairly uniformly distributed in thestomach so that representatives of all species always remainliving in a given stomach. Therefore this 'incidental selection'would not be decisive. The factor of adaptation to the denseenvironment is of greater importance, which adaptation in turnfavours the development of the skeleton and the caudal pro-jections, and these belong to the most characteristic criteria forthe differentiation of the Ophryoscolecidae. Therefore t h ea d a p t a t i o n here should be r ega rded as a mo t ivefac to r in t h e i r e v o l u t i o n . Since these Infusoria, how-ever, live in the greatest variety of different environments, i.e.in the greatest number of ruminants' stomachs, the oecologicf a c t o r s , too, must be considered. For instance, the sameconditions never exist in all those stomachs; the more theydiffer (on account of different food or some purely physiologicalcauses), the greater is the possibility of differentiation. How-ever, this oecologic factor is a r e g u l a t i v e one, not motive,as in all these various stomachs the adaptation as a reaction toall given special circumstances is of primary importance. Asfar as we know to-day, Ophryoscolecidae are transmittedthrough contact per os from ruminant to ruminant. This wouldbe in conformity with the already established fact that thosegroups of ruminants where this contact for any reason waspossible in the past but is no longer possible in the present, showa different structure of the species in their infusorial fauna.This is the factor of isolation. It, too, is a regulative factor.The isolation in a narrower sense (in the same locality, mechani-cal, if on account of difference in size no contact per os takesplace; in a small Cameroon goat and a camel, for example, keptin a zoological garden) as well as through geographic isolationmay cause a diversity in the several local faunas, simply becauseinfections and reinfections take place through contact per os.Here, in other words, we come to the conclusion that all factorsin the evolution are not equivalent at all, that they do notlie in the same dimension, on the same level.

DIPLODINIUM 47

Begarding P o l y p l a s t r o n and its 'caudopetality', thestructure of the skeleton and of the vacuolary apparatus is aresult of the motive factor of adaptation; while the fact thatin certain environments some of its varieties are more isolatedis a result of oecologie difference or of geographical isolationacting as regulative factors. Thus we can see clearly why P .m u l t i v e s i c u l a t u m was found to be predominant in Europe,the United States of America (primarily), and Northern Persia,while in Eastern Asia, in tropic regions, and in the United Statesof America1 P . h e g n e r i is predominant. Besides, P . m u l t i -v e s i c u l a t u m is a distinctive inhabitant of the stomach ofdomestic cattle, sheep, and goats, while P . h e g n e r i is charac-teristic (besides being present in domestic cattle and sheep) forB. b u b a l u s . Thus the evolution of a certain species dependsnot only on its particular but also on its other properties, andconsequently on the evolution of the species as a whole. Andthe evolution of the species as a whole is a consequence not ofany isolated evolutional factor, but of a number of factorswhich are interdependent and which act as a unity as cogs incogwheels. There are great chances of applying successfully toother forms this method of analysing the evolution of speciesderived from the study of these Infusoria, rightly held by allinvestigators to be of exceptional interest. For example, Beckerobserves that they are ' an assemblage of unicellular organismsunexcelled in complexity and diversity by any other ecologicgroups' (1932, p. 282). Owing to the great importance of theseInfusoria it is indispensable to first put in order their systematicsas a basis of all further cytological, biological, and physiologicalinvestigations.

SUMMARY.

In revising the species of the D i p l o d i n i u m (sub-genusP o l y p l a s t r o n ) the author comes to the conclusion that onthe basis of the generic criteria in the family Ophryoscolecidae,two distinct genera cannot be erected as was done by Kofoidand MacLennan (the genera P o l y p l a s t r o n and E l y t r o -

1 In the United States of America P. hegner i is a secondaryimmigrant (of Bos t au rus ) .

48 P. WEETHEIM

plast ron) . On the contrary it is justifiable to include bothof these species in P o l y p l a s t r o n (Dogiel, 1927) as a sub-genus of the genus Dip lod in ium. In conformity with theinternational rules of zoological nomenclature, the correctnames of both species in question are as follows: D. (Poly-plas t ron) m u l t i v e s i c u l a t u m Dogiel and Fedorowa,1925, D. (Polyplastron) hegner i Becker and Talbott,1927. Evidence is put forward to show how incorrect is theerection of Dogiel's aberrations P . m u l t i v e s i c u l a t u m'conf luens ' (monoscutum) and P . mul t ives i cu -l a tum ' f e n e s t r a t u m ' into separate species, and how theseforms are to be considered only as individual varieties. Theseviews are supported by the author's own investigations on liveand fixed (stained) material.

The author agrees with Kofoid and MacLennan that Poly-p l a s t ron had evolved from forms related to D. (Eudi-plodinium) affine, and brings forward arguments resultingfrom his investigations as to how this relation can be clearlydisclosed. He also gives a scheme of the systematic positionand phylogenetical relationship of the species in question ofDiplodinium—sub-genus P o l y p l a s t r o n .

Conclusions are drawn concerning the relation and kinshipof P . m u l t i v e s i c u l a t u m and P . hegner i , as well asthe author's rules in regard to the development of new con-tractile vaeuoles, also the reasons for the development of theskeleton and the position and form of skeletal plates. Theseskeletal plates serve for a general strengthening of the bodybesides other functions.

It is argued that a d a p t a t i o n is a mot ive factor inthe evolu t ion (the selection appears as an 'incidentalselection'), while the oecologic factors and the fac torsof the mechanica l and geographica l i so la t ion ofthe ruminant-hosts (in harmony with the per-os manner ofinfection of the ruminants by these Infusoria) act as regula-t ive f ac to r s . These two categories of factors are in diversedimensions (they are non-equivalent and inadequate). Theevolution of characters cannot be considered singly, but onlyspecies as units, and this evolution is not a result of isolated