462 



AURICULARIA. 



S.c 



anal and oral ridges are connected by two lateral ciliated bands, the 



whole forming a continuous band, 

 which, since the mouth lies in the 

 centre of it (fig. 255), may be re- 

 garded as a ring completely sur- 

 rounding the body behind the 

 mouth, or more naturally as a 

 longitudinal ring. 



The bilateral Auricularia is de- 

 veloped from a slightly elongated 

 gastrula with an uniform covering 

 of cilia. The gastrula becomes 

 flattened on the oral side. At the 

 same time the cilia become specially 

 developed on the oral and anal 

 ridges, and then on the remainder 

 of the ciliated ring, while they are 

 simultaneously obliterated else- 

 where ; and so a complete Auri- 

 cularia is developed. The water- 

 vascular ring in the fully-developed 

 larva has already considerably ad- 

 vanced in the growth round the 



TT-UlV/J-l iO O I I 1 I \_/WU.AJ.C;^IJ^VA *V J.l'J-1. Ul_LV/HJl>^-J.- _ //" lT\ ^ /I V 



vascular system; ic.v.r. water vascular oesophagus (fig. 2o6 W.V.r). 



p.v 



FIG. 256. FULL-GBOWN LARVA OF SYN- 

 APTA. (After Metschnikoff.) 



m. month ; sf . stomach ; a. amis ; 

 p.v. left division of peri visceral cavity, 

 which is still connected with the water- 



ring which has not yet completely en- 

 circled the oesophagus ; I.e. longitudinal 

 part of ciliated band; pr.c. prae-oral part 

 of ciliated band. 



Most Holothurian larvae, in their 

 transformation from the bilateral 

 Auricularia form to the radial form 

 of the adult, pass through a stage 



in which the cilia form a number of transverse rings, usually five in 

 number, surrounding the body. The stages in this metamorphosis 

 are shewn in figs. 256, 257, and 258. 



The primitive ciliated band, at a certain stage of the metamorpho- 

 sis, breaks up into a number of separate portions (fig. 256), the whole 

 of which are placed on the ventral surface. Four of these (fig. 257 A 

 and B) arrange themselves in the form of an angular ring round the 

 mouth, which at this period projects considerably. The remaining 

 portions of the primitive band change their direction from a longitudi- 

 nal one to a transverse (fig. 257 B), and eventually grow into com- 

 plete rings (fig. 257 C). Of these there are five. The middle one 

 (257 B) is the first to develop, and is formed from the dorsal parts of 

 the primitive ring. The two hinder rings develop next, and last of 

 all the two anterior ones, one of which appears to be in front of the 

 mouth (fig. 257 C). 



The later development of the mouth, and of the ciliated ridge 

 surrounding it, is involved in some obscurity. It appears from 

 Metschnikoff (No. 560) that an imagination of the oesophagus takes 

 place, carrying with it the ciliated ridge around the mouth. This 

 ridge becomes eventually converted into the covering for the five 



