STRUCTURE OF THE LARVA OF SPONGILLA LACUSTRIS. 409 



type B in cases in which the appearances figured are such as 

 would result from the fixation of individuals of that type. 

 But as type B is only a younger stage of type C, and not a 

 special variation of the fundamental type of larva^ so to speak, 

 no special description of the changes taking place in it appear 

 necessary. 



(1) Special Features of Type C during Metamor- 

 phosis. — It is characteristic of the metamorphosis of all the 

 types that the flagellated cells disappear more quickly from the 

 lower surface of the pupa than from the upper. However, 

 this feature is strongly emphasised in the pupa formed from 

 type C, as compared with that formed from type D. But 

 it is highly probable that this difference is not so great in all 

 the pupae derived from larvae of type C as it is in the one 

 actually represented in section in fig. 29. The structure of 

 the pupa in question is such that it is even possible that it 

 could thrive if the flagellated layer at the surface were 

 thrown off altogether, as described by Gotte. I have seen, 

 however, no evidence of such a procedure on the part of any 

 fixed larvae. In the case of the pupa figured, the flagellated 

 layer at the surface is as complete as in a free-swimming larva. 

 Both the nuclei and the flagella present the same appearance 

 as in the larva, and are equally well defined. On the other 

 hand, the flagellated cells which were at one time situated 

 at the lower surface have disappeared completely from that 

 position, and have migrated into the interior. They have be- 

 come plastered to the surfaces either of the cells with vesicular 

 nuclei, or of those with granular nuclei, as the case may be, 

 to form the plasmodial aggregations, which have already been 

 described in the previous section of this paper. The contrast 

 between the appearance presented by the flagellated cells 

 at the upper surface, and those which have at this stage 

 travelled to the interior from the lower surface, may be seen on 

 comparing figs. 29 a and 29 h, two figures drawn from the upper 

 and lower surfaces respectively of the same larva as fig. 29. 



Another feature characteristic of the pupa which results 

 frqm the fixation of type C, is the presence of fully developed 



