18/3.] DR. J. S. BOWERBANK ON CEYLONESE SPONGES. 31 



Its diameter very slightly exceeds that of one of the large spieula of 

 the sponge ; and its figure, full'of contortions, is just such as we may 

 imagine that of a dying, struggling, slender worm wonld have been 

 under such circumstances. It is so far covered and partially hidden 

 hy the surrounding sarcode, and so completely emptied of its former 

 contents, that nothing but its form is distinctly visible ; but he it 

 what it may it is certainly no part of the organization of the sponge. 



It is no uncommon event to find in living sponges such slender 

 worms attempting to prey either on the dermal surface or on the 

 interstitial membranes within the sponge. 



With such a structure as that of the specimen in course of descrip- 

 tion, a common investing dermal membrane could scarcely be sup- 

 posed to be required or to exist, and the true dermis must be sought 

 at the external surface of each of the skeleton-fistulae ; but as these 

 organs are so small and their parietes so thin, it is extremely difficult 

 to discriminate the dermal tissue from the parietes of the fistulse. 

 In some of the specimens mounted in Canada balsam, their surfaces 

 are frequently much obscured by sarcode ; but in some parts there 

 are strong indications of a very delicate dermal membrane envelop- 

 ing the fistula. 



Some of the large flecto-acuate spieula are procumbent and closely 

 adherent for the whole of their length to the surface of the fistulae. 

 They are disposed in lines more or less in accordance with its long 

 axis ; and in this position they perform the office of skeleton-spicula, 

 strengthening and supporting the delicately constructed fistula by 

 their longitudinal position on its surface, and maintaining it in its 

 normal position ; while others of them are projected as defensive 

 organs from its surface. Very few of the small incipiently spinous 

 defensive spieula are seen to be procumbent ; and by far the greater 

 number are projected backward at various angles to the surface of 

 the fistula. 



The direction in which both descriptions of defensive spieula are 

 projected from the surfaces of the fistulse is more frequently back- 

 ward than forward, or at right angles to the surface. This seems to 

 indicate that they are carnivorous feeders, and that this position of 

 the spieula is destined to obstruct the egress of any small annelids 

 that may have entered the body of the sponge, and to destroy them 

 for its own nutrition. 



No adventitious substances are incorporated with the skeleton of 

 this species as in the other two known ones, and all its spieula are 

 undoubtedly secreted by itself; and in every respect it forms the 

 best type of the genus Ilalijihijsema with which we are acquainted. 



DESCRIPTION OF THE PLATES. 

 Plate V. 



Bpongicmolla Holdncorthii. 



Fig. 1. The type specimen, natur.il size, exhibiting the numerous oscula within 

 the cup or upper surface of the sponge. 



