104 ADAM SEDGWICK. 



With regard to the endodermal organs the alimentary- 

 pouches have lost not only their connection with the alimentary 

 cavity and now constitute mesohlastic somites (fig. 8), but 

 have also lost their peripheral connection with each other. 

 The excretory pores persist and the part of the somites near 

 the pore becomes developed into the nephridia. 



In the other stock the body assumed a difi'erent shape, in 

 consequence of which the mouth and anus became terminal(vide 

 fig. 13, ideal). A projection overhanging the mouth then ap- 

 peared on the neural surface and gave rise to a neural praeoral 

 lobe (fig. 14.) The praeoral and postanal part (N^ and N'^) of 

 the nervous ring soon became inconspicuous and vanished. (It 

 must be remembered that the nervous system of this stage of 

 evolution was little, if at all, more developed than that of living 

 Actinozoa.) This is the stock of the Vertebrata and Balano- 

 glossus. The part of the primitive ring immediately behind 

 the mouth is the most important in this stock ; it is placed 

 at the anterior end of the body, and therefore enlarges and 

 develops sense organs. (Fig. 14.) 



With regard to the endodermal organs the pouches have 

 become differentiated into two kinds : 



(1) Anteriorly a certain number retain their communication 

 with the exterior and with the gut. (Fig. 10.) 



(2) The majority, however, lose their connection with the 

 gut and with the exterior, but remain connected by the peri- 

 pheral canal, which behind retains (by means of a pouch?) its 

 communication with the gut. 



(3) A posterior pouch loses its connection with the gut and 

 with the longitudinal canal, and gives rise to an abdominal 

 pore. 



The first group of pouches become the gill slits, the 

 second become the coelom, while part of each of them 

 become differentiated into nephridia which opens into the 

 longitudinal canal (pronephric or segmental duct). The last 

 pair of pouches gives rise to a part of the coelom and retains 

 its connection with the exterior as an abdominal pore. 



The further evolution of the Invertebrate Stock. — Paired 



