STEUCTURE OF THE LARVA OP SPONGILLA LAOUSTRIS. 473 



aggregations {pi. a.) are in an advanced stage of development, but tlie outline 

 of the flagellated cells adhering to their surfaces can be easily made out. 



Fig. 29 c. — Three fully developed collar-cells from the same larva as Fig. 29. 

 The cell a has a small nutritive vacuole (n. v.) which points to its recent origin 

 from a cell group, as well as three other small bodies, probably reduced yolk 

 bodies (y. b.). The nucleus (n. c. c.) is often onion-shaped, and the flagellum 

 which protrudes out of the collar passes down to it, and has usually a small 

 swelling at its base. 



Figs. 30, 30 «. — x 1000. These two flgures represent portions of 

 sections of an individual slightly more advanced than the one represented 

 in Fig. 29, derived from a larva of type C. Fig. 30 a is continuous 

 with the upper corner of Fig. 30. The flagellated layer has not yet com- 

 pletely disappeared from the surface, and its cells are becoming widely 

 separated owing to individual immigration, and also owing to the superficial 

 expansion of the pupa as a whole. The cells with granular nuclei {c.g.n.) 

 are in some cases at the surface, while in other cases they are seen in the act 

 of passing to it. At the lower surface (Fig. 30) these cells form an almost 

 complete layer. The larval cavity (Z. C.) is still very large, and is lined by a 

 layer of cells with granular nuclei similar to those at the surface, but smaller 

 and more flattened. The cavity shows no sign of disappearing. 



Tn Fig. 30 the nuclei of the flagellated cells are changing in character pre- 

 vious to the formation of plasmodial aggregations, the portion nearest the 

 margin being full of them. 



In Fig. 30 « a flagellated chamber derived from the cell groups has been cut 

 almost tangentially (C). 



Fig. 31. — x 1000. Portion of a section of an individual fixed for a 

 longer time than that drawn in Fig. 30. The flagellated layer has completely 

 disappeared, and the flat epithelium is well formed. The flagellated cells have 

 entered into the formation of plasmodial aggregations, and are indistinguish- 

 able from the yolk bodies. Flagellated chambers are in the process of develop- 

 ment from the cell groups, the collars and the flagella not having been produced. 

 This pupa here represented must have fixed during the stage of development 

 described as type B, or else the cells of the cell groups would have developed 

 collars before the fully formed condition of the plasmodial aggregations had 

 been attained. 



Fig. 31 a. — x 1000. Portion near the margin of a section of the same 

 individual as Fig. 31 . It shows how the flat epithelium of the upper and lower 

 surfaces passes into the marginal membrane, and how all kinds of cells make 

 their way into the cavity which exists between the two layers of epithelium 

 close to that membrane. 



