52 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



delicate branches with terminal orifices ; the two 

 vessels open into a special enlargement or bladder, the 

 walls of which are contractile, so that the fluid stored 

 up in it can be forced to the exterior. The sexes, as 

 in Nematoids, are ordinarily separate, and the males 

 can be distinguished from the females by their smaller 

 size. Rotifer, Brachionus, and Melicerta are examples 

 of the Rotatoria. 



Most of the forms with which we have already 

 had to do are small in size, and 

 it will have been noted that, where 

 the body attained, as in the case 

 of certain tape-worms, to a con- 

 siderable length, that body was not 

 an individual whole, but was broken 

 up into joints or segments. 



In the great group of worms 

 which we are now going to consider, 

 this segmentation of the body is 

 very distinctly exhibited, and affects 

 not only the external form, but the 

 great majority of the internal 

 organs ; this phenomenon becomes 

 the more comprehensible when we 

 learn that at one of its very earliest 

 stages in development the mesoblast 

 itself becomes regularly segmented. In. the simpler 

 conditions the segments, which we will henceforward 

 call metameres, are, for the greater part, exactly 

 similar in character, and only those at either end of 

 the body differ much from the rest. Later on we shall 

 see that, just as in the simpler animals, different parts 

 take on different duties, and division of labour becomes 

 as apparent among the metameres as it was in the 

 various persons of the colonial Coelenterata. 



Now, also, we find that organs for which, in the 

 smaller and simpler forms, there was no necessity, 



Fig. 19. Brachionus; 

 to show the Ciliated 

 head-disc. 



