Basic anatomy of metazoa

Peter Shaw

OverviewWe have already reviewed the standard taxonomic approach to invertebrate phyla - today we will examine the theoretical underpinning of the higher level classifications, with particular reference to body cavities. We will also examine other features of invertebrate body design. Remember: KingdomPhylumClassOrderFamilyGenusSpecies

The question today is how we organise this hierarchy

Within the metazoa, there are several different ways of classifying animals. Some are interesting but purely descriptive, such as a classification based on symmetry, or on skeletal systems.

Classification based on symmetry:

Radial – cnidaria (more apparent than real: really a 4-radial symmetry as in scyphozoa, or bilateral as in the internal anatomy of anthozoa) 

Pentagonal – echinodermata 

Bilateral – almost everything else (?Why?)

Classification based on skeletal systems:

No differentiated skeletal system: platyhelminths Mesogloeal tissue: many Cnidaria Hydrostatic skeleton: nematoda, and annelida (echinodermata, in addition to an endoskeleton). Complete exoskeleton: arthropods Partial exoskeleton: most molluscs, some cnidaria Endoskeleton: chordates (most), echinodermata.

Classification based on metamerism (segmentation)

Unsegmented – many phyla True segmentation (with all body characteristics repeated in each unit, at least in the primitive state)Annelids, arthropods, chordates Pseudo-metamerismRepetition of some parts of the bodyTapeworm bodies, stalked larval forms of scyphozoa (jellyfish)

Embryology

However, the most useful classification of body forms is that which is believed to best reflect evolutionary history. Often this has relied on embryology. Why?

This is because early embryonic stages differ far less between phyla than do adult forms. The early embryonic development seems to give us a glimpse into the development of long-lost ancestors.

SacculinaIn a few cases, the adult form of animals is so degenerate as to be unidentifiable, and before the advent of DNA-based techniques the only way top classify these oddities was by their early embryonic form. The classic example of this is the crab parasite called Sacculina. This exists as a fungus / cancer-like mass of undifferentiated cells permeating the whole of an infected crab’s body, emerging as a yellow sac at its genital opening. The larvae turn out to be identical to larval barnacles – they settle on crabs just like normal barnacles, then inject a mass of cells into the crab and cease to resemble any recognizable animal.

Haeckel’s dictum

 It is an old observation that embryonic development seems to re-trace evolution – embryonic humans resemble embryonic fish. In 1866 the German biologist Ernst Haeckel published a book titled Generelle Morphologie der Organismen, claiming that embryonic development retraced evolutionary history – giving rise to Haeckel’s dictum: Embryology recapitulates phylogeny This is not taken too seriously nowadays, but is still a nice quote.

2 or 3 cell layers?

The basics of embryonic development give us one fundamental division within the Metazoa. Some (presumably simpler) forms develop from 2 layers of cells, while the more complex forms develop from a 3-layered embryo. This gives us diploblastic and triploblastic life forms. Diploblastic animals have an endoderm (interior => guts) and an ectoderm (exterior => “skin”), but nothing else. These are the cnidaria and ctenophora – jellyfish and allies. Triploblastic forms have a third layer of cells, the mesoderm, which usually develops into muscles etc. (Oddly, in chordates the central nervous system develops from the ectoderm). All metazoan animals apart from cnidaria/ ctenophora are triploblastic.

Blastula – ball of cells Invaginates to make a gastrula, with2 or 3 cell layers

Ectodermendoderm

Ectodermmesodermendoderm

Diploblastic (cnidaria)

Blastopore(becomes mouth in protostomes, anus in deuterostomes)

Triploblastic (others)

Body cavities A next set of fundamental division is based on the development of body cavities during embryonic development. Most higher animals have fluid-filled cavities within the body. These allow space for organ development, allow for fluid circulation etc.

The simplest way to produce a body cavity is to retain the space between the ectodermal and endodermal layers of the embryo. This cavity is called the blastocoel, and is retained in most metazoa, giving a fluid-filled cavity variously called the haemocoel, pseudocoel or blood-vascular system. As the names imply, this cavity is often used to contain blood. In insects, molluscs, and many other invertebrates this is the only significant body cavity.

Coelom (pron. See - lom)

 Additionally, a second cavity can develop during embryonic development, arising de novo as a space between mesodermal cells. This is known as the coelom (or true body cavity), and is lined with a specialised layer of cells, the peritoneum. In mammals the coelom is the space occupied by guts, liver, heart etc.

Metazoa with a true coelom are known as coelomate. These include chordates, annelids, molluscs and echinodermata.

This gives us 3 divisions of animals, based on their body cavities: Acoelomate – no body cavity:Cnidaria, ctenophora, mesozoa, platyhelminths, nemerteans Pseudocoelomates – only with remnants of the blastocoel:Nematodes, rotifers, and various minor phyla (nematomorpha, gastrotrichs, entoprocts, acanthocephala + others) Coelomates: Fully developed coelom (though may be secondarily reduced):Molluscs, arthropods, annelids, chordates, echinodermata, + others

A final division within the coelomates is again based on embryology. Chordates and echinoderms have some common patterns of early development that differ from other coelomates, notably in the pattern of cell division and the formation of mesoderm + coelom. This leads to echinoderms, chordates (and a very minor group the hemichordates) to be classed together as deuterostomes, while the other coelomates are classed as protostomes.

Protostomes

PlatyhelminthsNematodesArthropodsMolluscsAnnelidsLophophorate phyla

Deuterostomes

ChordatesEchinodermsHemichordates

protostomes and the deuterostomes have different embryology.

Protostomes Deuterostomes

Cleavage of early egg: Spiral radial

Division Determinate Indeterminate (hence we can have identical twins)

Blastopore becomes mouth becomes anus

Coelom from within mesoderm pouch from gut wall

Chitin often present absent

c

No tissues: parazoa

DiploblasticCnidariactenophora

Acoelomate

Platyhelminthsnemerteans

Pseudocoelomatesrotifers, other minor phylaNematoda

Coelomates

Protostomes deuterostomes

ArthropodsMolluscsAnnelidsothers

Chordatesechinoderms

Triploblastic phyla

differentiated tissues: metazoa

DNA-derived phylogenies

Junk DNA – no selection pressure, varies quasi-randomly between individuals

Genome

Useful genes – can’t vary greatly within 1 species

Active site of crucial enzyme – changes hardly ever happen

An example of a crucial sequence that changes very slowly and may be used to derive high-level taxonomic relationships: the ribosome has to bid exactly to mRNA and to all the tRNAs or the organism will die before its first cell division. rRNA homologies are used to establish relationships between phyla.

DNA-derived taxonomy

We can now use these slowly-changing DNA sequences, notably 16srRNA, to derive an objective hierarchy for animal classification.

Generally it agrees well with the classical tree based on embryology, though there are a few changes, notably in that arthropods are joined with nematodes in the ecdysoza, while nemerteans and most platyhelminths join molluscs and annelids in a new group the lophotrochozoa (all having a prototroch larva).

DiploblasticCnidariactenophora

Platyhelminths? polyphyletic

Nematoda

deuterostomesArthropods Chordates

echinoderms

Triploblastic phyla

differentiated tissues:

metazoa

EcdysozoaLophotrochozoaMolluscsAnnelidsnemertines

DNA-derived classification of animal phyla

Myxozoa – once protozoan parasites of fish, now shown to be degenerate anemones!