OF THE SPONGIAD.E. 113 



apparent effect on the others. Nor is the habit of opening 

 and closing the oscula the same in every species. Thus in 

 the course of my observations on Halichondria panicea and 

 Hymeniacidon caruncula in their natural and undisturbed 

 localities, I have frequently observed during their exposure 

 to the air at low tide, that while no oscula in an open 

 condition could be found in Hymeniacidon caruncula, the 

 greater portion of those on the specimens of Halichondria 

 panicea were more or less in an open state. 



They appear also to be subject to a considerable amount 

 of modification as regards situation, even in the same 

 sponge. Thus in our common British species, Halichondria 

 pamcea, when of small size, they are situated on the surface 

 of the sponge, and are scarcely, if at all, elevated above the 

 dermal surface ; while in large specimens of the same 

 species we find them collected in the insides of large elongated 

 tubular projections or common cloacae, and these organs 

 vary from a few lines only in height and diameter to 

 tubular projections several inches in height, with an 

 internal diameter of half or three fourths of an inch. When 

 they attain such dimensions their parietes are often of con- 

 siderable thickness, and their external surface becomes an 

 inhalant one, like that of the body of the sponge. 



In many species the oscula are always elevated above 

 the dermal surface, and these thin pellucid elevations are 

 permanent, while in others, as in Spongitta fliiviatilis, the 

 tube exists only during the course of the energetic excur- 

 rent action ; and in such cases it appears to be subject to 

 great variation in size and form, as I have shown in the 

 description of Sponyilla in my " Further Report on the 

 Vitality of the Spongiadse," ' Reports of the British Asso- 

 ciation ; for 1857. 



INHALATION AND EXHALATION. 



The works of the old writers on Natural History are full 

 of vague opinions on the nature of sponges, but none of 

 them seem to have seriously studied their anatomy, or to 



8 



