STRUCTURE OF THE LARVA OF SPONGILLA LAOUSTRIS. 369 



may, nevertheless, be couveniently described apart from one 

 another. 



Type A. — In these larvae the inner mass consists of only two 

 kinds of cells ; namely, the cells with granular nuclei (a) and 

 with vesicular nuclei (/3), the latter alone making up the central 

 part of the solid posterior region (fig. 1). 



Type B. — The inner mass consists of three kinds of cell 

 elements above enumerated. The cells with granular nuclei 

 (a) are smaller and more numerous towards the interior. In 

 the cells with vesicular nuclei (j3) the " yolk bodies " are some- 

 what smaller. The " cell groups " are almost always in the 

 incompletely divided stage (figs. 5, 5 a, h, and c, and 9, 9 a 

 and b). 



Type C. — The composition of the inner mass is the same as 

 in type B, but with the following differences as regards the 

 development of the cells. The cells with vesicular nuclei (j3) 

 are much smaller than in types A and B, and contain fewer 

 "yolk bodies," and as a rule not more than one "nutritive 

 vacuole." The " cell groups " have the cells completely divided, 

 forming in many cases distinct flagellated chambers, and even 

 going so far as to develop collars and flagella (figs. 7 , 7 a, 11, 

 11 a, 12, 13, and 13 a and b). 



Type D. — The inner mass contains the same three elements 

 as in type B, but in very diff'erent proportions, and the cells 

 with granular nuclei (a) are reduced in size and much more 

 numerous. The " cell groups " (y) are exceedingly few in 

 number, though always present. The cells with vesicular 

 nuclei (/3) are much as in type C (figs. 6 and 6 a). 



The relationship of the four larval types is probably as 

 follows : 



Type A represents an early form, from which type B may be 

 derived directly by further differentiation of the cells of the 

 inner mass. Type C may be considered as a further develop- 

 ment of type B. Type D, on the other hand, must be regarded 

 as having arisen directly from type A. Two divergent lines of 

 development can therefore be distinguished in the larva, of 

 which the two culminating points are represented by types C 



