STRUCTURE OF THE LARVA OP SPONGILLA LAOUSTRIS. 431 



pointed. The raicroscleres, on the other hand^ are covered 

 with spines. The raegascleres form the main skeleton, and 

 have been for this reason called the skeletal spicules; while 

 the microscleres take no part in the formation of the main 

 skeleton, but are scattered about loosely in the sponge tissues, 

 especially in the epithelial membranes, and have consequently 

 been termed the flesh spicules. 



The Megaseleres. — These spicules always make their first 

 appearance in the cells with vesicular nuclei. They are found 

 in the very youngest free-swimming larva, and are placed at an 

 angle of less than 90° to the line which joins the two poles of 

 the larva. In the pupa, and subsequently in the young sponge, 

 they become arranged in various ways, but the majority of them 

 are placed almost vertically and slightly tilted towards the 

 periphery of the young individual. As they grow above the 

 general surface of the flattened or cake-like young sponge, 

 they carry with them the dermal membrane, which therefore 

 becomes raised up from the bulk of the tissues which lies 

 underneath. This lifting up of the dermal membrane on the 

 points of the spicules gives rise to the subdermal cavity. In 

 individuals which have been fixed for a number of days the 

 spicules, instead of appearing singly as at first, become aggre- 

 gated together to form fibres in which the axes of the spicules 

 are parallel to one another. They are surrounded by a layer 

 of cells which secretes a substance, presumably spongin, which 

 stains deeply red with fuchsin S. 



Figs. 36 a, b, c, and d illustrate successive stages in the 

 development of the megaseleres. At their first appearance 

 and for a considerable time afterwards they are enclosed 

 completely by the scleroblast, but what happens to the secreting 

 cell ultimately I have not been able to make out. The cyto- 

 plasm of the cell is exceptionally clear, but occasionally contains 

 a few yolk bodies and one or two nutritive vacuoles. So far 

 as my observations go, the nucleus of the cell does not divide, 

 but seems to lose its vesicular character, and to become 

 granular as the spicules increase in size. It is almost certain 

 that when a cell with a vesicular nucleus begins to secrete a 



