454 ItlOHAED EVANS. 



Conclusions. 



(1) That there are diflPeretit types of free-swiraraing larvae, 

 which have been described as A, B, C, and D. Type A is the 

 youugest form of all, type B is an intermediate form between 

 types A and C, while type D is a variation derived along a 

 different line of development from type A. 



(2) That the flagellated cells of the larva in all cases become 

 the collar-cells of the young sponge. 



(3) That certain cells of the inner mass, distinguished by 

 their vesicular nuclei and blastomeric characters, are capable 

 of giving rise to collar-cells, via the cell groups, as well as to 

 the flat epithelium, &c. ; i.e. both to the dermal and the gastral 

 layer. 



(4) That consequently, during the metamorphosis of type C, 

 flagellated chambers are developed from the " cell groups," 

 derived originally from cells with vesicular nuclei situated in 

 the inner mass, as well as from the flagellated cells. 



(5) That in type D hardly any cell groups are formed, and 

 consequently the gastral layer is developed almost completely 

 from the flagellated cells of the surface layer. 



(6) That both the cell groups and the flagellated cells of 

 the larva are to be considered as belonging to the same class, 

 the latter being developed on the outside, and consequently 

 producing only flagella, while the former originate inside, and 

 ultimately develop both collars and flagella. 



(7) That all the cavities, canals, and surfaces are lined by 

 cells possessing granular nuclei, which are capable of produc- 

 ing microscleres, even when they are situated in the flat epi- 

 thelium of the surface layers. 



(8) That the megascleres are produced at first in cells with 

 vesicular nuclei, which later on become granular. 



(9) That some of the cells with vesicular nuclei plaster them- 

 selves to the surfaces of the flagellated chambers in the young 

 sponge, and become pore cells, their nuclei subsequently chang- 

 ing in character and becoming granular. 



