Chap. III. GENUS PLEUKOBRACHIA. 237 



glass, Avlien the chances are frequent that the fringes will become folded directly 

 across the main stem. 



The outer wall {Fig. 15 {V /5" [>'", ^. 218) of the disk and the transversely striate 

 wall of the socket (/ /' y) are directly continuous the one with the other, and 

 together constitute the lining of the cavity which they embrace. The cells (/5 /•>') 

 of the disk wall are as broad as they are long, simulating irregularly polygonal 

 prisms, and the inner ends are flattened against the subjacent layer, whilst the 

 outer free ends are rounded. In size they are very large, being on the average 

 from one fifth to one fourth as long as the largest cells [Fig. 24) of the cellulo- 

 motor system. At the base (//) of the narrow ridge {g [/^ (f) they decrease in 

 size very rapidlj;-, and continue to do so until we come to the base of the tentacle 

 proper, at which point we find them diminished in diameter by nearly two thirds; 

 but from this place to the end of the stem and its fringes, the diminution is very 

 gradual, until at the tips {Fig. 13 c) of the latter they measure not more than 

 one fourth the diameter of those in the disk. The contents of these cells are 

 perfectly homogeneous, nor have we been able to see any mesoblast. During the 

 contraction of the fringes the surface is ribbed lengthwise, owing to the fact that 

 the outer wall folds upon itself {Fig. 18 c c^), and the inner one {d d}) projects 

 more or less into the dujolicatures. By taking advantage of the doubling of a 

 fringe upon itself, we may get a very satisfactory sectional view {Fig. 18) of these 

 walls when in this plicated condition. 



The outermost, or epithelial layer {Fig. 13 a h and Fig. 18 h), of the tentacidar 

 apparatus, is described below, and therefore need only be referred to in this con- 

 nection. Although the wall of the socket {j j^ f) is a very distinct layer, yet 

 it does not hang loosely, apart from the mass of cells which surround it, but 

 it is more like a lining to a cavity which has been excavated in the cellulo- 

 motor system. The transition from the comparatively thick outer wall of the disk 

 {Fig. 15 /:/) to the thin wall which constitutes the cellulo-motor system of the 

 sockets is very abrupt; but yet there is not so sudden a change in the nature 

 of the cellular constituents as would appear at first sight. The only appreciable 

 difference is in the shape of the two kinds of cells, the form of the discal cells 

 being adapted to a different purpose from those of the sockets. 



All the extensible parts of the tentacles, as well as their lateral fringes, are 

 covered by a layer of thick epithelial cells, every one of which is a lasso-cell} 

 When the tentacle is fully prolonged, these cells scarcely touch each other, and 

 then they display a perfectly rounded contour, excepting a very narrow portion, 



* The following investigation of the lasso-cells anJ colleague, Professor II. .J. Clark, who discovered 

 of Pleurobrachia is entirely the work of my friend their peculiar structure last year. 



