112 ANATOMY AND PHYSIOLOGY 



placed in a shallow pan of sea-water, and be allowed to die 

 of inanition, it then frequently expires with the whole or a 

 considerable portion of the pores open, and in that state it 

 may be readily preserved for the cabinet. 



THE OSCULA. 



The oscula are the fsecal orifices of the sponge. They 

 are situated at the distal terminations of the single or con- 

 centrated excurrent canals of the animal. They vary 

 considerably in form and size ; sometimes they appear as 

 single large orifices, while at others they consist of several 

 small orifices grouped together. When the sponges are 

 massive and solid, they are usually to be found dispersed 

 over the dermal surface, but occasionally they are grouped 

 on the highest portions or on the elevated ridges of the 

 mass. In Geodia Barretti, Bowerbank, MS., they are 

 concentrated in deep depressions or pits. In other cases 

 they are entirely hidden from the view, lining the interior 

 of elaborately constructed cloacae, situated in the centre of 

 the sponge, as in Grantia compressa and ciliata, Verongia 

 fistulosa, and a numerous series of species of fistulose 

 sponges from the West Indies. 



They are permanent organs, and are capable of being 

 opened or closed at the will of the animal, and are subject 

 to a considerable amount of variation in size and form, in 

 accordance with the variations in the actions of the sponge. 

 Thus in littoral sponges they are frequently entirely closed, 

 and their situation even quite indeterminable during the 

 period of their exposure to the air ; but when immersed in 

 water, and the sponge is in the energetic action of the imbi- 

 bition of nutriment, they are expanded to their full extent ; 

 but when this action ceases and that of gentle respiration 

 only exists, many of them close entirely, and others exhibit 

 apertures not exceeding half their former diameters, while 

 the imbibition of nutriment was in vivid action. Their 

 expansion or contraction is not rhythmical ; each can be 

 opened or closed at the will of the sponge without any 



