ORIGIN OP METAMERIC SEGMENTATION. 105 



processes of the body wall (fig. 10), into which the cavities of 

 the somites were continued are present (generally homologous 

 with tentacles of Coeleuterata which correspond with the 

 mesenterial chambers or radial canals). These become specially 

 locomotive, and consequently muscular; hence the swellings 

 (ganglia) on the nerve cords, each swelling corresponding to 

 appendages, i. e. to a somite. 



The septa between the pouches have more or less broken 

 down, so that the coelomic spaces become connected ; the 

 dorsal or ventral mesenteries, both or one of them, likewise 

 break down. 



Sometimes the appendages vanish (Gephyrea, Mollusca), 

 the ganglionic swellings then disappear, and the only trace in 

 the adult of the embryonic segmentation is seen in the 

 nephridia. Many of these must, however, have vanished 

 (according to Hatschek^s account of development of Echiurus), 

 and two or three or four pairs have become enlarged and alone 

 persisted. It is interesting to notice the differentiation of the 

 persisting nephridia in the Gephyrea into the brown tubes, 

 which act as excretory organs and generative ducts, and the 

 anal vesicles. This differentiation of the nephridia of different 

 parts of the body is carried, as we shall see, much higher in 

 the Vertebrata. In the Mollusca the disappearance of the 

 somites has gone even further than in the Gephyrea, and the 

 coelom has become much modified. In Nautilus, however, 

 a trace of the original segmentation persists in the nephridia 

 and vascular system. 



The development of Sagitta indicates that it is derived from 

 an ancestor with three pairs of pouches, two of which retain 

 their external pores (generative orifices). The Brachiopoda 

 I at present leave out of special consideration. 



Thus, the number of pouches (segments) in the Tripo- 

 blastica varies in different cases, just as do the alimentary 

 diverticula of the Actinozoa. 



The further evolution of the Vertebrate Stock. — The central 

 nervous system which is almost entirely derived from that part 



