Dr. J. E. Gray on the Growth of Hyalonema. 195 



acted on by the sea-water and the chemical constituents of the 

 mud. The discovery of this use of the spicules of Hyalonema 

 induced me to believe it might be the use of the long spicules 

 of Euplectella and Senvperella ; and more recently Mr. Carter 

 has shown that a Tethya is supported by similar elongated 

 flinty fibres. 



I must own that I am not convinced ; and I do not think 

 that I should be true to science and scientific truth if I did 

 conform to any views which do not satisfy my doubts, or I 

 should be most ready to give up my opinion if I were so, stand- 

 ing as I now do almost alone in my view of the question. 



It does appear to me remarkable that we should have 

 zoologists and physiologists of established reputation giving 

 so decided an opinion on the subject, when they do not con- 

 sider it necessary to reply to the reason that has been assigned 

 why the spicules of Hyalonema are not sponge-spicules. It is 

 true that Hyalonema and Sponges have siliceous spicules ; but 

 it is also shown that they occur in zoophytes, and that silica 

 forms a large part of the constituents of stony corals. The sili- 

 ceous spicules of sponges and Hyalonema have a central canal, 

 which Mr. Carter has lately shown is not found in the calcareous 

 spicules of sponges or zoophytes. The spicules of Hyalo- 

 nema are formed, like the axis of zoophytes, of concentric 

 layers ; but no microscopist or physiologist has attempted to 

 show me a siliceous spicule of a sponge that was formed of 

 concentric coats, nor have they responded to my challenge to 

 show me any spicule of a sponge that has the mode of growth 

 or the external microscopical characters of the spicule of Hya- 

 lonema, which as a spicule is mi generis, and is more like the 

 axis of a zoophyte than anything else. And why might not 

 a zoophyte have a bundle of axes as well as one ? They all 

 harp on the one string that the spicules of Hyalonema are sili- 

 ceous, and so are the spicules of most sponges, of all true 

 sponges (for I think the calcareous animal bodies that have 

 been called calcareous sponges belong to quite a different 

 class), and therefore Hyalonema must be a sponge — I must 

 say, a very lame conclusion when we consider how the sili- 

 ceous spicules of Hyalonema differ in structure and mode of 

 growth from the spicules of sponges. 



The zoologists and physiologists have not shown me any 

 sponge in which every spicule is surrounded by a regular 

 coat of sarcode. They say that this sarcode is full of siliceous 

 spicules of another form ; but why, if the sarcode existed and 

 formed one siliceous spicule, should it not form others of the 

 same or other forms? 



It has been objected that the Palythoa is so like the Palythoa 

 that does not secrete siliceous spicules, that it must be a para- 



