STUDIES ON THE COMPAEATIVE ANATOMY OP SPONGES. 17 



The stream of water leaves the flagellated chamber through 

 the gastric ostium or exhalant aperture^ a wide opening guarded 

 by a delicate^ membranous, sphincter diaphragm, as already 

 described (4) by Carter (vide figs. 23 — 26). 



In sections such as that represented in fig. 29 I have 

 not infrequently met with peculiar structures having the 

 appearance shown at x in the figure. These structures 

 evidently represent some probably normal phase in the 

 life-history of the flagellated chambers, and it appears to 

 me not improbable that they may be chambers in process of 

 dying. 



That the Sycon chamber, as an individual, can die, is perhaps 

 a somewhat novel idea; but if, as everyone will admit, an 

 Ascon individual dies, there is no difficulty in supposing that 

 a single chamber of a Sycon, which in many respects corre- 

 sponds to an entire Ascon, should also die. Indeed, it 

 is evident from the ontogeny of Grantia labyrinthica 

 (vide infra) that as the stalk develops the first formed 

 chambers must perish (fig. 27) ; and if so, why not indi- 

 vidual chambers later on ? Haeckel, in his work on the 

 ' Challenger Deep-sea Keratosa ' (12), suggests that the flagel- 

 lated chamber is to be regarded as the individual, comparable 

 to an individual person of a hydroid colony ; and in accord- 

 ance with this view we may certainly expect to find individual 

 chambers perishing, while the sponge as a whole continues to 

 exist healthily. 



In the cases alluded to (fig. 29, x), the skeleton of what I 

 believe to have been an originally normal and healthy chamber 

 is still present in exactly its normal position and enclosing 

 the normal space. This space, however, is no longer com- 

 pletely filled by the chamber, but the latter has shrunk away 

 from the surrounding tubar skeleton into the centre, where it 

 occupies little more than a third of its original diameter. The 

 remains of the collared cells are distinctly visible, closely, 

 if not exactly, resembling the collared cells which line the 

 surrounding chambers, only less regularly arranged and in 

 more than one layer. In the process of shrinking the meso- 



VOL. XlXII, PART I. NEW SER. B 



