STRUCTURE AND LIFE-HISTORY OF A SPONGE. 33 
of prismatic, or cylindrical cells, through them. They have, 
evidently, at this stage, lost their flagella. By excretion, or 
alteration, of the granular cells, a thin layer of jelly-like material 
is formed between the two layers of the embryo, this is the begin- 
ning of the future mesoderm, and in it the spicules as delicate 
needle-shaped forms first make their appearance. As they grow 
they soon enter the outer, or ectodermic, layer. 
The larva grows in the direction of its axis, and so elongates 
into a cylindrical or conical body, the distal end flattens, and 
becomes perforated by a hole which puts the stomach cavity into 
communication with the surrounding water. This hole is the 
adult mouth, the endodermal cells retreat from it, leaving the 
ectodermal layer alone around it, as a clear thin membrane 
terminating the gastral cavity. Little round spaces open in the 
side-walls of the sponge and form the pores, the endodermal cells 
acquire their characteristic collar and re-acquire their flagella. 
Simultaneously with these changes additional spicules appear. 
The double-pointed needle-like spicules projecting obliquely 
upwards and outwards, form a sort of tube, extending from the 
base to the summit ; immediately round the basal edge and the 
summital edge, they project outwards at right angles, forming a 
kind of collar. Mingled with the basal spicules are those with 
toothed ends before alluded to as grapnel-like spicules. Between 
the needle-like spicules are tri-radiate ones, all similarly arranged, 
two rays being directed, more or less concentrically, and the third 
longitudinally and downwards. Quadriradiates also make their 
appearance, and first round the terminal edge, two of their rays 
lying in the edge, one being directed longitudinally downwards, 
the other radiately towards the mouth, and serving as a 
support for the oral, or terminal, membrane. They are usually 4, 
6, or 8 in number at first, and are always symmetrically arranged. 
The description of the young sponge is so far complete ; it 
consists of a sac, with a mouth at one end and pores at the sides, 
Vol. III. p 
