Basic anatomy of metazoa

1. Basic anatomy of metazoa Peter Shaw

2. Overview We 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: Kingdom Phylum Class Order Family Genus Species 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. The question today is how we organise this hierarchy

3. 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?)

4. 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.

5. 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-metamerism Repetition of some parts of the body Tapeworm bodies, stalked larval forms of scyphozoa (jellyfish)

6. 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.

7. In 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. Sacculina

8. 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.

9. 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.

10. Invaginates to make a gastrula, with2 or 3 cell layers Blastula – ball of cells Blastopore (becomes mouth in protostomes, anus in deuterostomes) Ectoderm endoderm Ectoderm mesoderm endoderm Diploblastic (cnidaria) Triploblastic (others)

11. 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.

12. 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.

13. 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

14. 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 Platyhelminths Nematodes Arthropods Molluscs Annelids Lophophorate phyla Deuterostomes Chordates Echinoderms Hemichordates

15. 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

16. No tissues: parazoa differentiated tissues: metazoa Triploblastic phyla Diploblastic Cnidaria ctenophora Acoelomate Platyhelminths nemerteans Pseudocoelomates rotifers, other minor phyla Coelomates Protostomes deuterostomes Nematoda Arthropods Molluscs Annelids others Chordates echinoderms

17. DNA-derived phylogenies Genome Junk DNA – no selection pressure, varies quasi-randomly between individuals Useful genes – can’t vary greatly within 1 species Active site of crucial enzyme – changes hardly ever happen

18. 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.

19. 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).

20. DNA-derived classification of animal phyla differentiated tissues: metazoa Diploblastic Cnidaria ctenophora Triploblastic phyla deuterostomes Lophotrochozoa Molluscs Annelids nemertines Ecdysozoa Nematoda Arthropods Chordates echinoderms Platyhelminths ? polyphyletic

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