116 Prof. W. Thomson on the " Vitreous^' Sponges. 



among their meshes {Diplodemia &c.) ; so that I believe we 

 may regard the whole group as potentially siliceous. 



Dr. Schmidt's second family are the Ceraospongiae : — " Spon- 

 gise quarum sceletum formatur fibris solidioribus, recenti statu 

 plus minusve elasticis, quge stepius aliena corpuscula involvunt, 

 sed nunquam spicula in ipsis nata continent." 



His third family are the Gumminese : — " Spongias Corneo- 

 spongiis proximee. Parenchyma spississimum et maxime cona- 

 pactum, adspectu Kautschuk, quod tamen fibrillis tenuissimis 

 contextum est. Generum pars corpuscula silicea continent." 



The fifth family in Dr. Schmidt's arrangement are the 

 Halichondrise : — " Spongi^ spiculis siliceis pertextee, quae ob 

 telam laxiorem et minus spissam quamquam ssepius subcor- 

 neam neque Gummineis adnumerantur, neque Corticatis ob 

 defectum strati corticalis." 



The careful consideration of the diagnoses of these three 

 groups is quite as suggestive as the examination of an extended 

 series of the Sponges themselves, of a single graduated line of 

 forms in which there are no breaks of sufficient importance to 

 its subdivision into groups of higher value than fami- 



The horn Sponges and the Gummineae are so nearly allied 

 that they can be distinguished by comparative characters only. 

 The fibres of the horn Sponges are thicker, and the meshes 

 wider, the whole texture is more open than in the Gummineas, 

 in which the minute fibres are matted together in the consistent 

 sarcode, and the Sponge, when dried, looks like a piece of 

 leather. The general aspects of the two groups are very dis- 

 tinct; and even to the inexperienced eye the Gummineje form 

 a natural and easily recognizable series, whose characters it is, 

 however, scarcely possible to reduce to an intelligible defini- 

 tion. From the absence of positive characters, it is evident 

 that these two groups are liable at any point to pass into one 

 another. Among the Gummineae separate siliceous spicules 

 appear in abundance, the fibrillation of the horny matter 

 becomes obscm-e ; and we thus pass almost imperceptibly into 

 the fifth group, the Halichondrias. 



Professor Schmidt's fourth group are the Corticatae : — 

 " Spongi^ globosae vel tuberose ; spiculis siliceis pertextte, 

 peculiari strato corticali circumdatge, quod et tela organica 

 firmiori fibrillosa et plerumque corpusculorum siliceorum ge- 

 nere a parenchymate interiori differt." 



In this group we have positive characters of some value in 

 the very marked difference between the cortical layer and the 

 central mass, in the regular arrangement of the various histio- 

 logical elements, and in the peculiar type of the defensive 



