STRUCTURE OF THE LARVA OF SPONGILLA LAOUSTRIS. 379 



they bear to the number of other cells present varies consider- 

 ably. The size of the cell, which probably depends on the 

 number of divisions that have taken place, is far from constant, 

 even in the same larva. They become smaller as the larva 

 grows older, measuring from 8 to lO^u over one of their flat- 

 tened surfaces, but they are really too irregular in form to admit 

 of proper measurement. Sometimes a spiny microsclere appears 

 in these cells, even in the larva. 



The cytoplasm is usually clear, the cell body having at most 

 only a few enclosures. A "nutritive vacuole " is seldom seen 

 in them, but the " yolk bodies'' are often present, though few 

 in number.^ They also contain some of the small refringent 

 granules to which reference was made in describing the flagel- 

 lated cells (fig. 38 e). 



Tlie nucleus is either spherical or subspherical. Its usual 

 tendency is to assume the latter form when the cell is in either 

 the first or the second of the above-mentioned positions, and 

 the former when it is in the third. The above facts have no 

 importance other than that the general configuration of the cell 

 influences the shape of the nucleus, — that is, when the cell 

 becomes flattened the nucleus acquires a slightly compressed 

 form. The size of the nucleus, like that of the cell itself, and 

 probably for the same reason, is variable, ranging from 3^ to 

 5^ ju in diameter. 



The nuclear membrane is much thinner than that of the 

 nuclei of the flagellated cells, often to such an extent as to be 

 difiicult to distinguish from the surrounding cytoplasm. The 

 interior of the nucleus is occupied by numerous small and 

 irregularly shaped granules, placed at the nodes of a close and 

 fine nuclear reticulum. 



There is no reason that I can see to be found in the characters 

 of these nuclei whicii would justify the division of this class of 

 cells into two, namely, " epidermal cells" and '^ intermediate 

 cells " of Delage. They appear to be only one class of cells, 

 which are capable of being modified into flat epithelium 



1 For a further discussion of the " nutritive vacuoles " and " yolk bodies " 

 cf. Appendix A, pp. 422 — 425. 



VOL. 42, PART 4. — NEW SERIES. D P 



