342 Dr Grant's Observations on the 



roux, have described the hard glassy needles composing the 

 axis of the Gorgoma briareus, an animal which possesses re- 

 markably large eight-tentaculated polypi (Sol. & Ell. Cor. 

 p. 93.) Montagu has described these rigid asbestine or pumice- 

 like spicula in three of his species of British Sponges, Spongia 

 jpenwillus, S. verrucosa, and S. pilosa (Wern. Mem. vol. ii. p., 93. 

 et seq.) ; and the same hard glassy asbestine or silicious needles 

 have since been observed in different species of Tethya, Cydo- 

 nium, SpongiUa, Cliona, and in a great variety of marine 

 sponges. The extreme hardness of these spicula, however, is 

 the only character mentioned by preceding authors as indicating 

 silica to be their component earth. Not only the hardness of 

 these spicula, and their power of resisting heat, but also their 

 regular forms, their sharp points, and even their mode of ar- 

 rangement in several sponges, were known to our countryman 

 Ellis, who has described the spicula of the Spongia tomentosay 

 and represented their mode of grouping round the pores (Cor. 

 PL xvi. fig. D.), and mentions, that when they are burnt and 

 rubbed on the human skin, they pierce it, and excite an itching 

 pain. The regular forms of the spicula of several sponges have 

 been represented in the plates of Ellis, Cordiner, Lamouroux, 

 and they have been noticed by most writers, as Ellis, Gmelin, 

 Montagu, Lamouroux, and Lamarck, in their descriptions of 

 certain species. Donati had shown the example so early as 

 1750, by describing and representing the forms of the spicula, 

 and their mode of arrangement in other zoophytes (Storia Nat. 

 Mar. deir Adriat.) ; the useful example of Donati has been 

 rarely followed, but its advantages in the study of polymorphous 

 zoophytes will probably one day be more highly appreciated 

 from the constancy and preciseness of the character it affords. 

 The sponges in which I have found the spicula to consist of 

 silica, I have termed silicious sponges, to mark them as a sepa- 

 rate group, distinct from the horny and calcareous species al- 

 ready described. The spicula of silicious sponges are generally 

 smaller and simpler in their forms than the calcareous. They 

 can be more easily examined as the connecting animal matter 

 enveloping them can be completely removed by the blowpipe, 

 or by concentrated acids, without injuring their symmetrical 

 fonns, and they can thus be quickly obtained isolated for mi-^ 



