é ART. l.-r-î. UniA : IIEXACÏINÈLLIDA, ilt. 



The opening, witli a part of tlie gastral cavity into which it leads, 

 is seen in lig. 2, Pi. IL, near the upper edge and to the left of 

 the median line. It measures 36 mm. or more in maximum 

 diameter. The oscular margin is partly rounded and indistinct, 

 partly thin and wall-like. The gastral cavity may be described 

 as a shallow cup-like depression, not more than 15 mm. deep. 

 Its wall, formed of irregularly latticed strands, presents a some- 

 what powdery appearance, due to the presence of large numbers 

 of a certain hexaster. On it open a number of large and small, 

 oval or roundish, excurrent apertures, measuring up to 8 mm, in 

 width. Kight close to the apertures, the excurrent canals are 

 seen to freely and widely intercommunicate with one another ; 

 then, they penetrate deeply into the sponge-mass. — The osculum 

 and the gastral cavity here in question, I am inclined to assume, 

 were formed in a very early period of postlarval development. 

 For a time they must have given to the young sponge a cup-like 

 shape. With subsequent general growth, the cup-wall must have 

 increased so excessively in thickness as to have obliterated the 

 original shape, at the same time breaking through new oscula 

 on the one side, as the need arose. These secondarily formed, 

 or we may say, accessory oscula then seem to be equivalent in a 

 way to the parietal oscula of Euplectella, save in this unessential 

 point that they open into the excurrent canal-system, instead of 

 into a common gastral cavity. 



The secondary oscula are oval or roundish, and sometimes 

 rather irregularly shaped openings of various sizes. The larger 

 may measure 5 nun. or more across, while some are small per- 

 forations about 1 mm. or less in diameter. They are irregularly 

 distributed all over the rough-surfaced back, but somewhat more 

 sparingly on the upper rounded edge of the body. In some 



