456 FORAMINIFERA, POLYCYSTINA, AND SPONGES. 



yet been seen, belong to this last category. They have a pecu- 

 liarly simple structure, each being a sort of bag whose wall is so 

 thin that no system of canals is required, the water absorbed 

 by the outer surface passing directly towards the inner, and 

 being expelled by the mouth of the bag. The cilia may be 

 plainly distinguished with a l-8th inch objective, on some of the 

 cells of the gelatinous substance scraped from the interior of the 

 bag ; or they may be seen in situ, by making very thin transverse 

 sections of the substance of the Sponge. 1 It is by such sections 

 alone, that the internal structure of sponges, and the relation of 

 their spicular and horny skeletons to their fleshy substance, can 

 be demonstrated. In order to obtain the spicules in an isolated 

 condition, however, the animal matter must be got rid of, either 

 by incineration, or by chemical reagents. The latter method is 

 preferable, as it is difficult to free the mineral residue from car- 

 bonaceous particles by heat alone. If (as is commonly the case) 

 the spicules are siliceous, the Sponge may be treated with strong 

 nitric or nitro-muriatic acid, until its animal substance is dis- 

 solved away ; if, on the other hand, they be calcareous, a strong 

 solution of potass must be employed instead of the acid. The 

 operation is more rapidly accomplished by the aid of heat ; but if 

 the saving of time be not of importance, it is preferable on seve- 

 ral accounts to dispense with it. The spicules, when obtained 

 in a separate state, should be mounted in Canada balsam. 

 Sponge-tissue may often be distinctly recognized in sections of 

 agate, chalcedony, and other siliceous accretions, as will here- 

 after be stated in more detail (Chap. XIX). 



1 See Dobie, loc. cit. ; and Bowerbank in " Transactions of Microscopical Society/' 1st 

 Ser. vol. iii, p. 137. 



