36 SPONGES 



pour their contents, as the gastral cavity proper, from the excurrent canal 

 system. In many sponges, especially the thin-walled tubular or sac- 

 like forms, the gastral cavity is wide and sj)acious ; in others, especially 

 in marine or incrusting forms, it may be so much reduced by the thicken- 

 ing of the body wall as to be scarcely distinguishable from the exhalant 

 canals. 



It has already been seen (p. 23) that by folding or unequal growth of the 

 sponge, a false gastral cavity may arise, opening by a false osculum 

 (pseudosculum), and containing in its interior the true oscula which simu- 

 late the openings of the exhalant canals (cf. Fig. 35, B). Conversely, we 

 find in some Hexactinellids a flattening out of the gastral cavity and loss 

 of the osculum, in which case the openings of exhalant canals simulate 

 true oscula {Caulophacns, etc., Fig. 20). Hence it is not possible to 

 determine the nature of an excurrent opening by simple inspection, nor 

 even in many cases by its anatomical relations. 



Oscula are very often defended in various ways ; for instance, by 

 fringes or palisades of sharp spicules, or by sieve-like plates or mem- 

 branes across the opening (Figs. 15 and 18). In other cases the osculum 

 can be completely closed by a contractile sphincter or diaphragm 

 (Figs. 7 and 40). The oscular aperture may be on the level of the 

 general surface of the body, or raised up to form a special oscular tube, 

 often of great length (Figs. 25, 26, 31), according to the requirements of 

 the sponge. 



In the above account of the canal system a clear distinction has been 

 drawn between true pores and ostia. The former are found on the 

 surface only in the canal systems of the first type ; in other types the 

 inhalant openings are always ostia. The distinction is not, however, 

 always maintained, and superficial incurrent apertures are often loosely 

 termed pores, without reference to their true nature. 



Primitively. the ostia are scattered over the whole free surface. They 

 may be restricted, however, to the upper surface, Avhich bears also the 

 oscula, as in Tentorium (Fig. 31). In fan-shaped forms the ostia are on 

 one side, the oscula on the other, from which the condition witli a 

 pseudosculum and pseudogaster is readily derived. In the boring 

 forms of Cliona and its allies the sponge is embedded in a calcareous 

 matrix, but sends lobes up to the free surface, some of which 

 bear the incurrent openings, others the oscula. In many sponges the 

 ostia are aggregated into special sieve -like areas, termed pore sieves. 

 Upgrowth of the edges of such a sieve has probably given rise to the 

 remarkable state of things in Disyj'inga (Fig. 2G), the highest and most 

 specialised type of sponge so far as canal system is concerned ; a single 

 inhaliint opening leads by a long incurrent tube into a sort of atrial 

 cavity, surrounding the body of the sponge and containing what ajipear 

 to be the true ostia. Like the oscula, the ostia also may be defended by 

 spicules or by special contractile mechanisms, often reaching in Demo- 

 spongiae a high state of elaboration in the so-called chofus (see below). 



Many authors have sought to homologise oscula and pores, often 

 meaning ostia, however, by the latter term. True pores, as will be seen, 

 are distinct from oscula in that the former are intracellular, the latter 



