STRUCTURE OP THE LARVA OF SPONGILLA LACUSTRIS. 375 



several statements, which appear at present to be conflicting, 

 will be brought into line, and a more complete description 

 than has hitherto been given will result. 



The larvEe do not appear to hatch, or, at least, to swim out 

 of the mother colony, at the same age. It is quite possible 

 that they pass into the excurrent canals at approximately the 

 same stage of development, but owing to obstructions are 

 unable to swim out. They leave the mother colony by way of 

 the osculum, and are carried along by the current which issues 

 from that opening. They, however, soon gain control over 

 their movements, and swim to the surface of the water, darting 

 down instantaneously should they be disturbed in any way. 

 In all their movements the glistening broad end is directed 

 forward. When they reach the surface they either swim about 

 for a time, or settle almost immediately on the side of the 

 vessel, though they do not at first fix themselves to it. The 

 greater number will, if left undisturbed, fix to the side of the 

 vessel, while others will fix to the film of air at the surface of 

 the water. The interval of time between the actual swimming 

 out of the mother colony and the fixing is not constant. They 

 appear to fix in greater numbers between three and six o'clock 

 in the afternoon than at any otlier time. 



The larva is oval in form, or, perhaps more correctly, egg- 

 shaped, and is covered with a uniform coat of flagella. The 

 broader anterior end has a glistening appearance, owing to its 

 containing a large cavity, the "larval cavity ,^^ filled with a 

 kind of clear jelly. The narrower posterior end presents an 

 opaque appearance, owing to its being composed of cells which, 

 in the younger larvae, are full of food material, which, as deve- 

 lopment proceeds, becomes used up. 



In the following more detailed description of the cellular 

 elements of the larva the same plan will be maintained as was 

 followed in the abridged account given above. 



In the histological composition of the larva we can distin- 

 guish — 



(1) The flagellated layer at the surface (figs. 9 and 11). 



(2) The inner mass (figs. 9 and 11). 



