514 



COMPARATIVE ANATOMY 



CHAP. 



The Auricularia assumes the shape of a barrel. The circnmoral ciliated ring 

 atrophies in sixteen places, which are indicated in the diagram (Fig. 413). The 

 sixteen lengths of the ring which remain continue to grow and join, as indicated 



FIG. 409. Old Auricularia. 

 Transition to the barrel - shaped 

 pupa, the whole body decreasing 

 considerably in size. 1, The nerve 

 bands, in the act of forming the 

 nerve ring ; 2, primary tentacle. 



FIG. 410. Intermediate stage be- 

 tween Auricularia and the barrel- 

 shaped pupa of Synapta (after 

 Semon). I-V, the rudiments of the 

 five ciliated rings. 1, The oral funnel ; 

 2, the primary ; 3, the secondary out- 

 growths of the water vascular ring ; 

 4, pieces of the calcareous ring ; 5, 

 coelomic vesicle ; 6, water vascular ring. 



by dotted lines (Fig. 413), to form five ciliated rings entirely encircling the barrel - 

 shaped body. The centre of the former oral area becomes surrounded by four lengths 

 of the ciliated band which join together to make a square. The part of the oral 



FIG. 411. Young barrel - shaped 

 larva (pupa) (after Semon). 1, Oral 

 funnel ; 2, tentacles ; 3, pieces of the 

 calcareous ring ; 4, Polian vesicle ; 

 5, left coelom ; 6, hind-gut ; 7, audi- 

 tory vesicles ; 8, secondary out- 

 growths of the hydrocoel ring. 



FIG. 41-2. Barrel- shaped larva, 

 with the tentacles (1) beginning to 

 project from the opening oral funnel 

 (after Semon). 2, Water vessels of 

 the body secondary outgrowths of 

 the circular canal ; 3, the rapidly 

 swelling enterocoel. 



area enclosed by this ring sinks below the surface, and thus increases the size of the 

 oral vestibule. The ciliated square itself becomes depressed and forms the oral 

 shield. The spacious oral vestibule becomes cut off from the exterior, with the 

 exception of a very narrow aperture, and shifts quite to the front, so that now the 



