6o PROPAGATION OF MARINE SPONGES. PART in. 



The spicula and skeleton of most of the marine 

 sponges are siliceous and singularly beautiful; the 

 skeleton of the Dactylocalyx pumiceus of Barbadoes is 

 transparent as spun glass ; and a species from Mada- 

 gascar has numerous simple transparent and articu- 

 lated spicules implanted in the siliceous fibres of the 

 skeleton. The Cristata, Papillaris, Ovulata, and many 

 more have siliceous skeletons, some garnished with 

 spicules of various forms, and the surface occasionally 

 covered with a layer of siliceous granules. 



The variety in the size, structure, and habits of the 

 marine sponges is very great : temperate and tropical 

 seas have their own peculiar genera and species ; some 

 inhabit deep water, others live near the surface, while 

 many fix themselves to rocks, sea-weeds, and shells, 

 between high and low water mark. There are very few 

 dead oyster, whelk, scallop, and other shells that escape 

 from the ravages of the Cliona, an extremely minute 

 burrowing sponge of the simplest structure, which has 

 a coat of siliceous spicules supposed to be the tools with 

 which it tunnels a labyrinth through the mid-layer of 

 the shell, in a pattern that varies with the species of 

 the sponge. A communication is formed here and 

 there with the exterior by little round holes, through 

 which the sponge protrudes its yellow papillae. From 

 the force exhibited by this little sponge, it may perhaps 

 be inferred to possess a rudimentary muscle and nerve. 5 



Sponges are propagated twice in the year by minute 

 ciliated globules of sarcode, detached from the interior 

 of the aquiferous canals, which swim like zoospores to 

 a distance, come to rest, and lay the foundation of new 

 sponges. The little yellow eggs of Halichondria pani- 

 cea are lodged in the interstices between the interior 

 canals ; when mature, they are oval and covered with 

 cilia, and are carried out by the currents ; and after 



5 Professor Huxley's Lectures. 



