446 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The parietal canal is enlarged in such a way that its walls, without increasing 

 perceptibly in mass, continually become thinner. 



The beginnings of the definitive gut develop from the two median dorsally 

 and ventrally running horns of the anterior sack and from the central section in 

 so far as it is not concerned in the formation of the hydrocoele and the parietal 

 canal. With the dissolution of the union between the two ccclomic sacks the two 

 vertical entodermic processes unite, first anteriorly and later also posteriorly. 



During these changes the gut in different individuals shows striking vari- 

 ations in the thickness of the walls and in the extent and form of the lumen 

 which can not be explained as part of a continuous developmental series. 



Cross sections show the gut predominently triangular witli the base of the 

 triangle parallel to the hydroccele sack and the apex dorsal, lying between the two 

 ccelomic sacks of the mesentery. It does not run exactly in the major axis, nor 

 parallel to it, for its anterior end is displaced toward the left and its posterior 

 usually a little toward the right. 



The walls are composed of cylindrical or cubical cells, among which on the 

 outer side are occasionally seen globular cells in process of division. 



It is certain that the parietal canal and the water vessel intercommunicate 

 after they have severed their connection with the definitive gut. The last point 

 of union with the gut is at tlie region of the connection between the two, so that 

 they might be considered as two evaginations of unequal size from the anterior 

 gut section extending dorsally and ventrally from it, while the two ca-lomic sacks 

 might be considered as right and left evaginations of the posterior end of the 

 gut, near the blastopore. 



It might be said that in the primitive gut (arclienteron) after its closure 

 there can be differentiated three divisions — a posterior (lying near the blasto- 

 pore), a middle, and an anterior. The posterior has already been separated off 

 and divided before the anterior appears. The middle remains single and forms 

 the definitive gut. The anterior and posterior each divide into two parts; the 

 latter gives rise to the right and left ca?lomic sacks, the former ventrally to the 

 hydroccsle and dorsally to the parietal canal. 



The mesenchyme filling the primitive body cavity is now composed of more 

 numerous but smaller elements, rounded or star shaped, with short processes. 

 The more or less gelatinous albuminous substance which in the younger stages 

 filled the body cavity has been almost entirely consumed by the mesoderm cells. 



In the crinoids the division of the archenteron is as follows : 



f peritoneal sac fright coelome. 



(at hinder end) | lef t ccelome. 



[ mesentero-liydrocoele 

 In the other echinoderms: 



f mesenteron. fdefinitive hydrocoele. 



[primitive hydrocoele sac [parietal canal. 



i , ... , , . , f peritoneal 



[one or two vaso-peritoneal sacs (at anterior end) < , , , 



' liydrooflple 



[peritoneal sac. 

 sac. 



