382 RICHARD EVANS. 



and encloses a variable number of small granules situated at 

 the nodes of the nuclear reticulum. 



(y) The Cell Groups. — These groups of cells are situated 

 in the interior of the solid posterior part of the inner mass. 

 The number of cells which constitute a group is highly 

 variable. The groups seem to be present in all the older 

 larvse^ though they are far more numerous in some individuals 

 than in others. They are so few, however, in some older 

 larvae that they might have been overlooked had it not been 

 that in some individuals they are more numerous. In some 

 cases the groups are well defined and isolated, while in others 

 they run into one another owing to their close proximity. 

 Sometimes they are surrounded by a well-developed membrane, 

 formed, apparently, by the cells with granular nuclei, this 

 being especially true of the groups situated near the centre of 

 the solid posterior end of the inner mass. In the youngest 

 larvae in which the groups occur the cytoplasmic bodies of 

 the individual cells cannot be distinguished from one another; 

 but in older larvae the cells are perfectly independent, and may 

 go so far, even in the free-swimming larva, as to form collars 

 and flagella. In the latter case they enclose a cavity, which 

 is that of the flagellated chamber. Just as the cells them- 

 selves exhibit a progressive development from an incompletely 

 divided condition to one in which the cells are free from one 

 another, so also the nuclei pass through a series of changes. 

 At one stage they resemble those of the flagellated cells to 

 such an extent that the same description might be made to 

 apply to both, while at a later stage they assume the definitive 

 characters of the nuclei of the collar-cells.^ 



Now that a general description of the cell elements which 

 enter into the histological composition of the larva has been 

 given, it is necessary to discuss the relative quantities, occurring 

 in the diff'erent types of larvae, of the elements above enume- 

 rated. Four main types of larvae may be distinguished, which 



' For a further discussion of the possible origin of the cell groups cf. 

 Appendix B, pp. 422 — 425. 



