Studies on the Development of Larvai Nephridìa. 503 



siderable readjustmeut of the relatiouships of the various organs in 

 the larva has taken place. This is brought about priucipally by 

 the outgrowth of the anal regiou, and the formatiou of the tentacles. 

 These last arise in pairs, towards the dorsal suiface, the most dorsal 

 pair always being the youugest. This is also the case, as Cald- 

 WELL pointed out, with the tentacles of the adult, but the adult 

 tentacles at the same time grow laterally, new pairs arisiog both 

 veutrally and dorsally to the first pair, so that the most ventral of 

 the adult tentacles are not necessarily the oldest. Increase in size 

 seems to correspond with the number of tentacles and their size, as 

 Mastermax has shown. About this time the cuticle of the young 

 larvae becomes very opaque and granular, and this added to the 

 greater rapidity of movement acquired by the larva vs^hen it has 

 attained two pairs of tentacles make it very difficult satisfactorily 

 to follo w the internal changes taking place in liviug material. The 

 larvae roll themselves up in a ball on the slightest irritatiou. The 

 rapid outgrowth of the tentacles also adds to the diftìculty as they 

 render it impossible to obtain satisfactory side views of the animai. 



By referring to pl. 31 fig. 18, it will be seen that the body of 

 the larva ean be roughly divided into a pre-orai lobe, and a main 

 trank region. The p re -or al portion consists of a flexible hood 

 which is often carried folded down over the mouth, on the trunk 

 region. Into the hood, the blastocoelic cavity of the truuk extends. 

 being crossed in all directions by the processes of the mesenchyma- 

 tous cells which are quite numerous in this portion of the cavity. 



The trunk region is composed of the main portion containing 

 T the stomach. and a posterior anal papilla, containing the gut or 

 rectum, the nephridia, and the true coelom; and is separated from 

 the stomach portion but faintly, by the line of outgrowth of the 

 tentacles. Thus the body cavity of the pre-orai hood and the trunk, 

 exclusive of the space of the true coelom partially surrounding the 

 gut, is blastocoelic in character. With the growth of the larva the 

 true coelom, which is at first a small sac on the dorsal side of the 

 gut, increases at the expense of this blastocoelic cavity, until in 

 advauced stages the only remaining part of it is the celiar 

 ■-pace, and the cavity of the hood. The c oliar cavity ^ which has 

 given rise to so much discussion, is certainly not coelomic, as 



1 The coUar cavity of Mastermak, not the true coelomic cavity of this 

 region, the preseptal cavity. 



