SECT. ii. FRESH- WA TER SPONGES. 6 1 



swimming about for some days fix on a solid object, 

 become covered with bristles, spread out into a trans- 

 parent film, charged with contractile vesicles of different 

 sizes in all degrees of dilatation and contraction, as 

 well as with sponge ovules. Spicules are developed at 

 the same time, and these films ultimately become young 

 sponges, and if two happen to meet they unite and are 

 soldered together. 6 Besides eggs, larger bodies covered 

 with radiating spicules are produced, containing granu- 

 lar particles of sarcode, each of which when set free 

 by the rupture of the envelope, becomes an Amoeba- 

 like creature, and ultimately a sponge. 



Fresh-water sponges are sometimes branched, and 

 sometimes spread over stones, wood, and other sub- 

 stances ; and one species covers an earthy mass some 

 inches thick formed by its own decayed matter. The 

 skeleton of such species as have one, consists of bundles 

 of siliceous spicules, held together and mixed with 

 groups of needles, the rods of which project through 

 the surface of the sponge and render it spinous. The 

 motions in the gelatinous sarcode mass are the most 

 remarkable feature in the fresh- water sponges, which all 

 belong to the genus Spongilla. Mr. Carter observed 

 that portions of the surface of some individuals of the 

 Spongilla fluviatilis in his aquarium had long cilia by 

 means of which they rapidly changed their places 

 during the spring, but when winter came they emitted 

 processes from such parts of their surfaces as were free 

 from cilia and retracted them again just like Amoebae. 

 These portions often had cells, and when the Amoeba- 

 like motions ceased, a nucleus and nucleolus appeared 

 within them, and at last the whole gelatinous sarcode 

 mass consisted of these cells or globules. Some had no 

 nucleus, but were filled with green or colourless granules. 



At certain seasons of the year, whatever the form of 



6 M. Milne-Edwards. 



