28o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



GENERAL CONCLUSIONS. 



The general conclusions of this chapter may be 

 briefly summarized in several propositions. 



i. There are at least four very distinct plans of 

 structure among animals through which homology may 

 be more or less clearly traced, but beyond which it can 

 not be distinctly traced, although it doubtless exists. 

 These are the vertebrata, articulata, mollusca, and radi- 

 ata. The characteristic plan of vertebrates is that of 

 an internal skeleton, the axis of which consists of seg- 

 ments ideally similar, but modified according to the place 

 and function in the series of segments. The animal 

 thus formed is again modified according to its place in 

 the scale of vertebrates. 



The characteristic plan of the articulata is that of an 

 external skeleton, composed of segments and pairs of 

 appendages, ideally similar, but modified according to 

 the place in the series. The animal thus formed is 

 again modified according to its place in the scale of 

 articulata. 



The characteristic plan of the mollusca is the total 

 want of a true locomotive skeleton, and especially the 

 entire absence of segmentation. There is therefore no 

 room for serial homology. Even special homology is 

 more obscure in this than in other departments. Nev- 

 ertheless, there is enough to assure us that these also 

 came by modification from some primal form of mol- 

 lusk. 



The characteristic plan of radiata is that of similar 

 segments arranged symmetrically about a central mouth 

 and stomach. In vertebrates and articulates we have 

 segments repeated in a linear series ; in radiates in a cir- 

 cular series. In all other types we have bilateral symme- 

 try ; in this we have radial symmetry. 



