Prof. W. Tliomson on the " Vitreous " Sponges. 115 



of Sponges from monographs and memoirs in which external 

 form is chiefly considered, such as the beautiful work of 

 MM. Duchassaing and Michelotti, and Johnston's ' British 

 Sponges.' 



The microscopic characters of Sponges, derived from the 

 structm-e and form of the reticulated supports, and from the 

 special forms of the spicules, are usually well marked ; but 

 these characters are in most cases of specific, in a few of ge- 

 neric, and in scarcely any of ordinal value. Fortunately, the 

 chemical composition of the organs of support and of defence, 

 two or three well-defined general types of structm-e and form 

 of the spicules, and the general arrangement and mode of 

 combination of the different parts, present some good points of 

 distinction for larger groups. 



I think that certainly the most satisfactory arrangement of 

 Sponges is that proposed by Dr. Oscar Schmidt *. Dr. Schmidt's 

 memoir, however, labours under the disadvantage of dealing 

 with the Sponges of the Adriatic only; so that many remark- 

 able exotic forms, which might have modified to some extent 

 his ideas of classification, are excluded. In the second Supple- 

 ment, indeed, the author institutes a comparison between the 

 Adriatic Sponges and those described by Dr. Bowerbank ; but 

 the series is still incomplete, from the absence of illustration 

 from the rich fauna of the East and West Indies. 



Dr. Schmidt divides the Adriatic Sponges into six groups, of 

 the rank of families t- The first of these are the Calcispongiae, 

 Avhich he thus defines : — "Spongi^e parvse, plerumque albi- 

 cantes, corpore spiculis calcareis pertexto." 



It seems to be generally admitted that the Sponges with 

 calcareous spicules are essentially distinct ; and I am inclined 

 to agree with Dr. Gray|, who places them in a distinct subclass, 

 antithetic to the whole of the remainder of the Sponges, which 

 form in his arrangement a second subclass under the name of 

 POEIPHORA SiLICEA. 



There is an evident awkwardness in placing such genera as 

 Spongia and Halisarca (in which there is no silica whatever 

 in any separate form) among the siliceous Sponges ; still I 

 think the classification is justifiable; and it is at all events 

 convenient. The true horn Sponges pass by almost imper- 

 ceptible gradations into the groups which develope distinct 

 siliceous elements, either within the fibres (e. g. Chalina) or 



* 'Die Spongien des Adriatischen Meeres,' Leipsic, 1862 (and two 

 Supplements). 



t Handbuch der vergleiclienden Anatomie. Jena, 1865. 



X "Notes on the Arrangement of Sponges," Proc. Zool. Soc. Lond. 

 May 9, 1867. 



