DEVELOPMENT LARVAL FORMS 



605 



a.c.o.b 



rudiment of the "coelom" or secondary body-cavity of the larva ; 

 the remainder of the archenteron forms the definitive gut, and 

 becomes divided by constriction into an oesophagus, a stomach, 

 and an intestine, and at the same time bent into a shallower or 

 deeper V-shape, the concavity of which is towards the concave 

 side of the body. Within this area of the surface a new 

 funnel-shaped depression makes its appearance. This is the 

 " stomodaeum," the rudiment of the mouth of the larva, and it 

 soon joins the apex of the larval oesophagus ; the conjoined tubes 

 henceforth bearing the name oesophagus since the ectodermal 

 and endodermal parts become indistinguishably fused. Along 

 the sides and floor of the oesophagus is formed a V-shaped ridge 

 bearing strong cilia ; this is the 

 " adoral band of cilia " which 

 sweeps the food (consisting of 

 Diatoms, Infusoria, etc.) into the 

 mouth. The larva is now known 

 as a Dipleurula and appears in 

 four modifications, each character- 

 istic of a Class of Eleutherozoa. 

 These differ from one another prin- 

 cipally in the following points : 

 (a) The folding of the ciliated 

 band ; (b) the divisions .of the 

 coelomic sacs ; (c) the develop- 

 ment and fate of the praeoral lobe 

 (i.e. the part of the body in front 

 of the mouth) ; (d) the fate of 

 the larval mouth. The types of 

 Dipleurula are as follows : 



(1) The Bipinnaria, the larva 

 of Asteroidea. In this type there 

 is a very long prae-oral lobe. 

 The ciliated band runs along its 

 edges, and is produced into a 

 backwardly directed loop on its 

 under surface. This loop soon 

 becomes separated from the rest 

 of the band as a distinct prae-oral 

 loop, the rest forming a post-oral loop. Both loops are drawn 



a.d.a. 



.jj.d.a. 



.....pl.a. 



Fig. 283. Bipinnaria of Luidia. a,. 

 Anus ; a.b, adoral ciliated band ; a.c.o.b, 

 anterior median process ; a.d.a, an- 

 terior dorsal process ; a.v.a, prae-oral 

 process ; vi, mouth ; p.c.o.b, median 

 dorsal process ; p.d.a, posterior dorsal 

 process ; p.l.a, posterior lateral pro- 

 cess ; p.v.a, post-oral process. (After 

 Garstang.) 



