STRUCTUEE OF THE LARVA OP SFONGILLA LACUSTRIS. 397 



takes place during and after fixation, it will be necessary to 

 consider two extreme cases which will be the result of the 

 settling down to a sessile life of types C and D. 



As far as possible the following arrangement will be pur- 

 sued. First, the features common to both types will be 

 described ; secondly, the special features of the two types will 

 be considered — in the first place of type C, and in the second 

 place of type D. The formation of the cavities and canal 

 systems in general will be followed in the subsequent pages. 



The larva described above as type B, so far as its changes 

 need be considered, can be taken along with type C, for they 

 both present in common the important feature of possessing 

 cell groups, — either as small cells more or less incompletely 

 divided, or quite independent of one another, but not pro- 

 vided with collars and flagella ; or as flagellated chambers 

 in which the cells are adorned with collars and flagella, and 

 have attained their definitive arrangement. There is also 

 another difference between types B and C when they are 

 about to fix themselves, which will be of necessity the cause 

 of a variation occurring in the structure of the young fixed 

 stages. The difference in question is the presence of a great 

 number of yolk bodies in the cells with vesicular nuclei in 

 type B, while in type C they are always few, though invari- 

 ably present. This is an important feature, the bearing of 

 which is only rightly appreciated when it is recognised that 

 there is a stage during the metamorphosis at which it is 

 almost impossible to distinguish the nuclei of the flagellated 

 cells from the yolk bodies. 



The larva described above as type A need not be taken into 

 consideration, as it probably never fixes itself at so early a stage 

 in the development. Type D, on the other hand, presents 

 extremely important features which must be specially described. 

 It agrees with type C to the extent of possessing cells with 

 vesicular nuclei which contain but few yolk granules. It 

 must be admitted, however, that there is nothing impossible in 

 a larva fixing itself which was in an earlier stage than type D, 

 but situated on the line of development from types A toD, and 



