ECHINODERMATA. 



555 



-2>.v 



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 meta- 

 morphosis, 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 pro- 

 jects considerably. The remain- 

 ing portions of the primitive 

 band change their direction from 

 a longitudinal one to a trans- 

 verse (fig. 257 B), and eventually 

 grow into complete rings (fig. 

 2570). 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 invagination 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 tentacular outgrowths of the water- vascular ring (fig. 

 258), and possibly also forms the nervous system. 



The opening of the cesophageal invagination is at first behind 

 the foremost ciliated ring, but eventually comes to lie in front of 

 it, and assumes a nearly terminal though slightly ventral position 

 (fig. 258). No account has been given of the process by which 

 this takes place, but the mouth is stated by Metschnikoff (though 



FIG. 256. FULL-GROWN LARVA OF 

 SYNAPTA. (After Metschnikoff.) 



m. mouth ; st. stomach ; a. anus ; 

 p.v. left division of perivisceral cavity, 

 which is still connected with the water- 

 vascular system ; w.v.r. water-vascular 

 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. 



