STRUCTURE OF THE LARVA OF SPONGILLA LAOUSTRIS. 403 



centres of attraction in possessing stores of food material. At 

 this stage it is almost impossible to distinguish the nuclei 

 of the flagellated cells from the yolk bodies always present in 

 greater or less number. 



The cell aggregations compounded each of numerous flagel- 

 lated cells, and one cell with a vesicular nucleus or a granular 

 nucleus, as the case may be, must be carefully distinguished 

 once and for all from the " cell groups " of type B, in 

 which the cells were incompletely divided from one another, 

 and never had a separate existence. The groups here de- 

 scribed will be referred to as " plasmodial aggregations," for 

 they follow in their formation the principle involved in the 

 building up of a plasmodium rather than that which has to be 

 considered as connected with phagocytic action. In the former 

 case no cell is subordinated to the other in any way, but in 

 the latter one cell takes in the other, and seeks to destroy it. 

 From the bionomie point of view the action of the flagellated 

 cells in this case is comparable rather with that of the com- 

 mensal, which feeds, as it were, at the table of another, but 

 does not directly harm the host. Similarly the flagellated cells 

 feed on the food material which has been stored up in the 

 cells with vesicular nuclei, and which they inherited from the 

 egg cell in the course of their development. If the formation 

 of these groups were a case of phagocytic action the large 

 central cell would have to be considered as the phagocyte, 

 taking in all the flagellated cells it could lay hold of, and 

 endeavouring to absorb and destroy them. This view has 

 been put forth, but, as will be seen, it obtains no support from 

 the subsequent development of the plasmodial aggregations. 



The outline of the plasmodial aggregations is at a certain 

 stage as well defined as tliat of the cells with vesicular nuclei 

 in the youngest larva, that is in type A. During the pupal 

 life there is a stage, however transitory, during which the 

 young individual consists of only cells with granular nuclei 

 at the surfaces and plasmodial aggregations inside, provided 

 always that such an individual has been produced from the 

 larva described above as type D (figs. 16, 16 a, 26, and 27). 



