12 PRIMITIVE ANIMALS 



with regular epithelial walls ; this cavity, the coelom, 

 forms the general body or perivisceral cavity into 

 which the viscera and other organs are projected 

 (Fig. 3). Besides this cavity, which contains a special 

 fluid of its own, the coelomic fluid, another sepa- 

 rate system of canals is formed in the mesoblast, 

 quite distinct and disconnected from the coelom ; 

 these are the blood-vessels, which contain a defi- 

 nite fluid of nutritive and respiratory function, the 

 blood. 



Finally, we have the nervous system concentrated 

 into a dorsally situated brain above the mouth, con- 

 nected by a pair of commissures with a ventrally 

 situated, segmented, double nerve-cord. 



Here then, in this immense animal phylum, we 

 meet with at least four characteristics of prime 

 importance, the presence of limbs, the metameric 

 segmentation, the development of a coelomic body- 

 cavity and of a separate vascular system, and the 

 ventral segmented nerve-cord ; but as to the origin 

 of any of these characters we are really in the dark 

 and can only vaguely speculate as to the mode of 

 their acquisition. 



The Mollusca or Shell-fish constitute another 

 great phylum of the animal kingdom, the com- 

 ponent groups of which have been shown by the 

 industry of naturalists to be reducible to a similar 

 plan of organization and therefore to constitute a 



