FRESH-WATER SPONGES. 185 



the cells, is mucilaginous. When observed in the delicate pellicle, 

 which, with its embedded cells and granules, it forms over the surface, 

 and throughout the canals of the sponge, it is transparent ; but when a 

 portion of this pellicle is cut from its attachments, it collapses, and 

 becomes semi-opaque. In this state, the detached portion immediately 

 evinces a tendency to assume a spheroidal form ; but whether the in- 

 tercellular substance participates in this act, or remains passive while it 

 is wholly performed by the habit of the cells which are imbedded in it 

 to approximate themselves, Mr. Carter has not been able to determine. 

 The seed-like bodies occupy the oldest or first-formed portions of 

 the sponge, near its periphery. They are round or oviod, according 

 to the species, and each presents a single infundibular depression on its 

 surface, which communicates with the interior. At the earliest period 

 of development in which Mr. Carter recognised the seed-like body, it 

 has been composed of a number of cells, united together in a globular 

 or oviod mass, according to the species, by an intercellular substance 

 similar to that just described. The seed-like body passes from the 

 state just mentioned into a more circumscribed form; then becomes 

 surrounded by a soft, white, compressible capsule ; and finally thickens, 

 turns yellow, and develops upon its exterior a firm crust of siliceous 

 spicula. 



Thus matured, its cells, which were originally unequal in size, now 

 become nearly all equal, almost motionless, and a little exceed the 

 average diameter of the largest sponge-cells ; while their germs, which 

 in the first instance so nearly resembled the gemmules of the sponge- 

 cells, are now four or five times larger ; and vary in diameter below the 

 3000th part of an inch, which is the average linear measurement of the 

 largest of their kind. 



The capsule passes from a soft, white state, into a tough, yellow, 

 coriaceous membrane, presenting in meyeni and plumosa a hexagonally- 

 tessellated appearance, on the divisions of which rest asteroid disks of 

 the vertically- placed spicula which surround it. 



On the development of Spongilla Mr. Carter remarks : " When the 

 cells of the seed-like body are forcibly expelled from their natural 

 cavity under water, they are irregular in form and motionless, but soon 

 swell out (by endosmose?), become globular, and after a few hours 

 burst. At the time of bursting, their visible contents, which consist of a 

 mass of germs, occupying about two-thirds of the cavity of the cell, sub- 

 side, and afterwards gradually become spread over the bottom of the 

 vessel in which they are contained. They are of various diameters 

 below the 3000th part of an inch, which is the average linear measure- 



