512 



COMPARATIVE ANATOMY 



CHAP. VIII 



it has itself separated from the archeiiteroii (Fig. 403). In the last case, it can be 

 established that the archenteron wliioh begins with the blastopore opens outward for 

 a time through a second aperture, the primary madreporite. 



After the hydro-enteroctulomic vesicle lias become constricted from the archen- 

 teron, the intestine grows further, its blind end bending to the ventral side (that lying 

 opposite to the water pore), which commences to become depressed and to sink in. 



The blind end of the intestine soon becomes applied to the ectoderm of the 

 depressed ventral side of the larva, about half way down the body, or a little in 



aji 



Fig. 405.— Young Auricularia of 

 Synapta, from th^ ventral side (after 

 Semon). 1, Circumoral ciliated band ; 

 2, entero - hydrocrnl ; 3, calcareous 

 wheel : 4, adoral ciliated ring ; os, 

 mouth; an, anus; 5, mid -gut or 

 stomach intestine ; 6, larval nerve 

 band. 



Fig. 406. — Older Auricularia, seen diagonally from the 

 left lower side (after Semon). 1, Circumoral ciliated band ; 

 2, hydropore ; 3, hydroco3l ; 4, adoral ciliated band ; 5, mid- 

 gut or stomacli - intestine ; 6, nerve band; 7, hind -gut; 

 S, left enterocoel ; 9, calcareous wheel ; 10, fore-gut, reso- 

 phagus ; 11, right enterocoel. 



front of the middle point. Where the two touch one another, an aperture, the mouth, 

 breaks through. 



The median portion of the intestine (the mid-gut) swells up and becomes distinct 

 both from the fore-gut and from the hind-gut. 



In the meantime, the larva has undergone a change of shape through which it 

 reaches the Auricularia stage, the depression of the ventral side being the most 

 important part of this change. The general ciliation has disappeared ; of the 

 complete covering of cilia, only the circumoral ring and the adoral band are 

 retained, and the region immediately around the mouth has become depressed to 

 form the oral vestibule (Figs. 404-407). 



The transformation of the Auricularia into the barrel-shaped larva (Figs. 408- 

 413). — The Auricularia does not change direct into a young Holothurid, but passes 

 through an intermediate stage, which was formerly known as the pupal stage, 

 because during it no nourishment is taken. 



