STUDIES ON THE COMPARATIVE ANATOMY OP SPONGES. 13 



pose to retain in the present paper, because, although the 

 gastral cavity corresponds functionally to the oscular tube of 

 siliceous and horny sponges, it is very improbable that the 

 two structures are homologous. 



The Inhalant Pores. 



According to Haeckel (7), in certain of the Sycons a portion 

 of the inflowing water obtains direct access to the tubular 

 flagellated chambers by means of " dermal ostien " situate at 

 the distal extremity of the latter. This never takes place in 

 Grantia labyrinthica, although Carter maintains, as I 

 have already mentioned, that the pore-sieves are placed over, 

 and lead into the distal ends of the chambers. As a matter of 

 fact, the pore-sieves, or pore-areas, lie between the ends of 

 the flagellated chambers, and over the ends of the inhalant 

 canals. The blind ends of the flagellated chambers, on the other 

 hand, are covered over by the well-developed dermal cortex. 



Each group of pores — for which we may conveniently use 

 the term pore-area as in other sponges — is usually more or 

 less oval in outline, and contains a dozen or more small round 

 pores (fig. 30) . The longer diameter of the pore-areas averages 

 about 0'33 ram. in length, and the diameter of the pores 

 themselves about 0'033 mm. The pore-areas and pores are 

 best studied in tangential sections of the dermal surface, a 

 method the importance of which cannot be too strongly insisted 

 upon. In the pore-areas the cortex is reduced to a mere thin 

 membrane, corresponding to the dermal membrane of other 

 sponges (e.g. Monaxonider), containing large numbers of 

 small oxeote spicules, and perforated by the pores. If the sec- 

 tion be very thin — only of about the thickness of the dermal 

 membrane — it is not easy to determine the boundaries of the 

 pore-areas, which lie very close together. If, however, the 

 section be fairly thick, then a portion of the mesodermal 

 trabeculse separating the subdermal cavities will be included, 

 and the appearance shown in fig. 30 will be presented, where 

 each pore-area is seen overlying the end of an inhalant canal. 

 Pig. 31 shows a single pore more highly magnified. 



