STRUOTUEE OF THE LARVA OF SPONGILLA LAOUSTRIS. 465 



condition of the larva on the one hand, and the method of preservation with 

 Hermann's fluid on the other. 



Pig. 56. — x 1000. Section of the solid posterior end of the inner mass, 

 containing a cell group {t/. c.) in which the nuclei lie at the periphery, the 

 cytoplasm being incompletely divided. There is also at the top of the cell 

 group a curious structure, perhaps the remains of the old nucleus which 

 gave rise to the smaller nuclei. 



Fig. 5 c. — x 1000. Section of a cell group from the same larva as Fig. 5. 

 In the centre is seen a structure which apparently represents the nutritive 

 vacuole, surrounded by small nuclei. 



Fig. 6. — x 1000. Portion of a section at the side of the larval cavity, 

 type D. The ceils are devoid of enclosures of any kind. The cells with 

 granular nuclei have become quite flattened to form a lining to the larval 

 cavity. Their nuclei are slightly compressed, but otherwise do not diff'er from 

 those of the cells which lie between them and the flagellated cells. 



Fig. 6 «. — X 1000. Part of a section of the same larva as Fig. 6. Note 

 that the cells with granular nuclei {c.ff. n.) are far more numerous than they 

 were in the larva represented in Fig. 5 a, type B, and that the " cell groups " 

 are far less numerous, being in reality almost a negligible quantity. The 

 yolk bodies (y. b.) are also greatly reduced in size and diininibhed in numbers. 

 All the cells in general as well as their nuclei are far smaller than they are in 

 either type A or type B. 



Fig. 7. — X 1000. Portion of a section of type C at the junction of the 

 larval cavity with the solid posterior part of the inner mass. Note the cells 

 with granular nuclei {e.g. «.) making their way towards the surface, and the 

 cell groups [g. c.) in places arranged in chamber-like rings with developing 

 collars. The yolk bodies are few in number, and stain red owing to the 

 method of preservation. The nutritive vacuoles are intensely black owing to 

 the action of Hermann's fluid during preservation. 



Fig. 7 a. — X 1000. Two chambers drawn from the same larva as Fig. 7. 

 One chamber {A.) has been cut transversely, while the other {B.) has been 

 cut longitudinally. The collar-cells have well-developed collars and flagella, 

 and are arranged to form horseshoe-like figures. The large opening towards 

 which the collars and flagella are directed is the apopyle (or exhalant pore). 

 It opens into the beginning of an exhalant canal {E. C), which is lined by a 

 flattened cell with granular nucleus. Note that the collars unite by their 

 margins to form what is usually called SoUas's membrane. 



Fig. 8. — X 1000. Surface view of the osculum [osc.) and ostia {ost.) 

 of a young sponge soon after their formation. The osculum is still on a level 

 with the general surface. The rim is formed of superposed layers of cells 

 with granular nuclei, which form the flat epithelium. Note also the presence 

 of an ostium (?) close to the osculum. It is a small opening measuring about 



