SPONGES 



95 



take place chiefly in one of two ways. First, in compact forms the whole 

 sponge may be enveloped in a sort of outer covering or skin, termed a 

 pseudoderm, formed by outgrowths from the Ascon tubes situated most 

 peripherally; as a consequence the primitively wide and irregular en- 

 trances between the outermost tubes into the intercanal system become 

 reduced to small orifices termed pseudopores. Secondly, the intercanal 

 system may become greatly enlarged towards the centre of the sponge, 

 forming a false gastral cavity or pseudogaster. In consequence of these 

 modifications of the intercanal system the sponge may secondarily 

 assume the form of an Olynthus, well seen in the species Clathrina 



PP 



FIG. (if.. 



Canal system of Clathrina ventricosa, Crtr., seen in vertical section, psd, pseudoderm ; Ps.G, 

 pseudogaster ; osc, oscula ; i.e., intercanal system ; pp, pseudopores (r.c. on the right, should be 

 i.e.). Schematised after Dendy. 



ventricosa, Carter (Fig. 66). Here, however, the apparent pores are really 

 pseudopores (pp) leading into the intercanal system (i.c), and the apparent 

 gastral cavity is a pseudogaster (Ps.G), opening by a pseudosculum. The 

 true oscula (osc) open into the pseudogaster, and the wall of the vasiform 

 sponge is made up of the coiled Ascon tubes. A pseudoderm (psd) is 

 formed towards the cavity of the pseudogaster as well as towards the 

 exterior of the body wall. The two species Clathrina ventricosa and 

 tripodifera offer striking examples of homoplasy, since a very similar form 

 and structure is arrived at in perfectly different ways, and the large 

 central cloacae, with their excurrent orifices, are not in the least homo- 

 logous in the two forms. 



