60 OPINIONS AND DISCOVERIES 



the zoophytes, and for referring them to the Algae. It is true 

 that the structure of sponges varies much from that of other 

 algae, but the structiu-e of the latter presents already in 

 its class such remarkable modifications that it does not sur- 

 prise us to meet one more among them. Grant has ob- 

 served in sponges spicula of pure silex corresponding very 

 well to the slender fibrous points that we see in the Spon- 

 gilla, and that have also equal solidit}" and tenuit}'. * 



The sponges have also engaged the attention of Du- 

 jardin, a micrographer of great reputation and experience, 

 but who seems to have had few opportunities of examining 

 any other than the fresh-water species. He is right, I 

 think, in asserting that the skeleton of the sponge is, under 

 every modification, a product of secretion, and not of crys- 

 tallization, as the language of Audouin and ]Milne-Ed- 

 wards seems to imply. He recognizes the thi-ee modifica- 

 tions of that skeleton described by Grant, but he asserts, if 

 I do not misinterpret his meaning, that the calcareous and 

 siliceous spicula may co-exist in the same sponge : and he 

 controverts the position that the fibres of the horny sponges 

 ai'e tubulai', for, on the contrary, they ai'e full and imper- 

 forate. The spicula are formed by the successive deposi- 

 tion of layers, and are mostly solid also, though in several 

 there is the appearance of a longitudinal canal in the cen- 

 tre, and in others small ca\"ities are visible. Those of most 

 common occurrence ai'e smooth and fusiform, sometimes a 

 little arched, one-fifth of a millimetre in length, and from 



* Annales des Sciences Nat. n. s. Part. Bot, ii. p. 328—30. From 

 his last remark, Link would seem to be ignorant of the siliceous nature 

 of the spicula of Spongilla. His Alcyonium paniceum is also evidently 

 a sponge, identical with our Halichondria panicea. 



I 



