THE MORPHOLOPxY OF TRK CM KILOSTOMATA. 271 



ceived with scepticism,, wliich proves to have been un- 

 justified. 



The Compensation-sag of this species is a very large cavity 

 which underlies pi'actically the whole of the frontal surface 

 of the zooecinm. In longitudinal section (fig. 26, c. -s-.) it is 

 seen to be perfectly distinct from the body-cavity, from 

 which it is sepai-ated by a delicate membrane, constituting 

 the floor of the sac. This membrane passes continuously 

 into the base-line of the operculum, immediately proximal to 

 which the sac opens to the exterior. When the operculum 

 is closed the aperture of the sac is a virtual transverse slit. 

 In the case of ordinary Escharine forms this aperture is so 

 little apparent that it has commonly been supposed that the 

 base-line of the operculum actually articulates with the 

 adjacent part of the calcareous wall — a state of affairs which 

 is often by no means the case. The roof of the compensation- 

 sac is protected by the calcareous bars (/. h.), and the frontal 

 wall of the zooecium is bevelled oif on the proximal side of 

 its aperture, in the characteristic way shown in fig. 2(3. The 

 purpose of this arrangement is obvious. The sharp edge of 

 the frontal wall of the zocecium is in contact with the base- 

 line of the operculum when the latter is closed. When the 

 operculum is open (fig. 24) the plane of its free surface 

 becomes parallel with that of the bevelled edge of the frontal 

 wall, so that water can have unobstructed access to the sac. 

 As supposed by Jullien, there can be little doubt that the 

 enti'y of water into the sac renders the protrusion of the 

 polypide possible, but it almost necessarily follows that the 

 constant change of the water in the sac makes this structui-e 

 an important organ of respiration. 



In this species I have been able to obtain only unsatis- 

 factory evidence, in consequence of the state of its preserva- 



indicate tliat he had appreciated the morphological importance of his discovery. 

 The words are as follows : — " Muscles retracteurs de rabdonien (fibres muscu- 

 laires parietales des auteurs). Elles rctractent en realite la paroi al)dominalc, 

 mise en I'videnco par la docouverle de la cliamhre ;\ can de compensa- 

 tion ou chambre compcnsatrice." 



