CONCLUSION 309 



In most of the coelomata, the mesoblast contains a definite 

 cavity, the coelom, separate from the alimentary cavity. 



The various examples of Coelomata described in the present 

 volume fall into five main groups or phyla, according to 

 their several plans of organisation. Thus, Distomum is an 

 example of the phylum Platyhelmia ; Lumbricus is an example 

 of the phylum Chaetopoda ; Anodonta and Helix are examples 

 of the phylum Mollusca; Apus and Astacus belong to the 

 class Crustacea, Periplaneta to the class Insecta, both 

 classes being included in the phylum Arthropoda. Lastly, 

 Amphioxus^ Scyllium, Rana, and the Mammalia are examples 

 of the phylum Chordata. There are other phyla included 

 in the division Ccelomata, but as no examples of them have 

 been described in this book, they need not be discussed here. 



When we speak of a " phylum," we mean an assemblage 

 of animals whose structure and organisation may be referred 

 to a particular plan. Thus in the phylum Chordata, the 

 rabbit, the frog, the dogfish, and Amphioxus, much as they 

 differ from one another in detail, resemble one another in 

 certain essential points, such as the dorsal position and 

 tubular nature of the central nervous system, the possession 

 of an axial skeleton lying between the nervous tube and 

 the gut, the ventral position of the main contractile blood- 

 vessel or heart, the existence of myotomes, etc. This much 

 can be learnt from a study of their adult anatomy, but 

 their embryonic anatomy teaches us something more namely, 

 that certain structures which are absent in the adults of the 

 higher members of the phylum are nevertheless present in the 

 embryos of all the members of the phylum, though they persist 

 in the adults of the lower forms only. The gill-slits are an 

 excellent example of this phenomenon. They are important 

 functional organs in Amphioxus and Scyllium, conspicuous 

 and functional in the tadpole, but aborted in the adult frog, 

 and present, but in a functionless condition, in the embryo 

 of the mammal. Further than this, we learn from embryology 

 that other important structures, such as the arteries and 

 some of the cranial nerves, are moulded, so to speak, on 

 the gill apparatus, and that the original pattern is carried 

 over into those adults in which all traces of gill-slits, as 

 such, are lost. Clearly, then, the possession of gill-slits must 

 form a part of the structural plan which we conceive of as 



