496 E. A. MINCHIN. 



coriacea, aud the ring-like sphincter which I described in 

 clathrus, as well as similar structures in other species^ are 

 made up of the porocytic epithelium of the interior of the 

 oscular rim — a strong proof of the contractile nature of these 

 cells. I may repeat an earlier statement of mine, namely, that 

 when contracted the epithelium lining the oscular rim never 

 assumes the mushroom form characteristic of the epithelial 

 cells of the exterior when in a similar condition. 



Do the porocytes multiply amongst themselves? I have 

 stated above that they do not, which means I have never seen 

 them do so, at least not when fully differentiated ; while on the 

 other hand 1 have, I think I may say, abundant proof of their 

 origin individually from ordinary epithelial cells. But PI. 41, 

 fig. 38, shows a cell of the porocytic epithelium of the oscular rim 

 containing two nuclei, proving that they may multiply in this 

 region. I have never found functional pore-cells multiplying 

 to form more pores, though it would be rash to affirm too 

 positively that they could never do so, but at least it is a rare 

 occurrence. To sum up briefly the results of this investigation 

 upon the pore-cells, we have found — 



(i) That the porocytes form a definite and well-characterised 

 layer of cells. (2) That their characteristic features are the 

 same as those of the ordinary dermal epithelial cells, but greatly 

 exaggerated. (3) That they arise by modification and trans- 

 formation of ordinary dermal epithelial cells, which come to lie 

 amongst the collar-cells in two ways : at the oscular rim by 

 inclusion amongst the collar-cell layer as it grows upwards; 

 elsewhere by actual immigiation from the surface into the 



there is no trace of eitlier sieve membrane or sphincter. In others again, 

 incrusting forms of opaque, leathery appearance, with the tubes forming a 

 network in one plane, the whole gastrai cavity is traversed by a network 

 running in all directions, not only at the oscular rim, but everywhere. This 

 network has the same structure as the sieve membrane, and its threads are 

 made up entirely of granular porocytes surrounding an axis apparently of 

 jelly. The specimens which show this network are very resistent and firm 

 even when exjianded, and no doubt derive support from it. Compare the 

 similar network figured by Dendy in " Leucosolenia" proxima (1891 

 [2], pi. viii, figs. 1, 2). 



