SPONGES 49 



incline to regard them as intercellular gaps simply, formed by the 

 epithelium of the incurrent canals dipping in towards an interval 

 between the collar cells. On the other hand, the dermal openings of the 

 incurrent canals in Demospongiae have frequently been described as 

 intracellular ducts, especially in very young specimens (Carter, Maas, 

 Delage). It is possible that the openings seen in these cases were those 

 of true porocytes belonging to chambers in direct contact with the outer 

 surface. That the ostia of the incurrent canals should be formed by 

 intracellular perforations of porocytes would be a fact very difficult to 

 interpret in the light of the general evolution of the canal system, as 

 sketched above. 



It seems, on the whole, more reasonable to assume at present, until 

 the contrary has been proved, that in siliceous sponges also true (intra- 

 cellular) pores are to be found at the prosopyles. In that case the 

 prosodus would probably owe its origin to the elongation of a porocyte 

 and its duct, and variations in this respect would explain the contra- 

 dictory statements made in some cases (e.g. Oscarella) as to whether prosodi 

 are present or not. The question is one, however, which cannot be 

 settled without further investigation. 



While the existence of intracellular pores, comparable to those of 

 Ascons, is doubtful in siliceous sponges, there seems no doubt that cells 

 comparable to the porocytes exist in a variety of siliceous sponges 

 the so-called cellules splieruleuses of Topsent (in part ; see below, p. 

 59). The cells in question are of lobose, amoeboid form, densely packed 

 with refringent granules, which obscure the nucleus ; they resemble, in 

 fact, the contracted porocytes of Ascons. They are very conspicuous cells, 

 and it is therefore remarkable that their pore-forming function, if they 

 possess any such, should not have been observed hitherto ; precisely the 

 same fact was, however, true of Calcarea until quite recently, porocytes 

 having often been observed, but their relation to the pores overlooked. 1 



In many siliceous sponges some of the very granular cellules 

 sphtruleuses, which are here regarded as porocytes, secrete fibrils 

 of an elastic substance differing so little in its nature from the 

 spongin that cements the spicules, that it can only be regarded as 

 a variety of it (Loisel [10]). The cells in question, which may 

 be termed spongoblasts, are found sometimes isolated, sometimes 

 in groups, but most commonly in rows, like a string of pearls (Fig. 

 51, A, a and b). The spongin makes its appearance near the 



1 A possible theory of the porocytes would be that they were cells of the dermal 

 layer which in some cases have acquired a special ingestive or phagocytic rfilt in 

 addition to their other functions. Such cells would naturally tend to place 

 themselves near the openings through which the currents enter the sponge body, 

 and might eventually come to surround these apertures. This view would explain 

 not only the alleged differences, mentioned above, in the position of the pore cells in 

 calcareous and siliceous sponges, but would also explain their apparent absence in 

 many of the latter, where it must be supposed that the ingestive mechanisms remain 

 at a lower stage of elaboration. In support of the theory here put forward, it may 

 be pointed out that the porocytes of Calcarea entrap and ingest larger bodies, such 

 as Diatoms, which are often to be found in them. 



