Chap. III. 



GENUS PLEUROBRACHIA. 



235 



{Fig. 15 (f), two chymiferous tiiljes (cc a a" a") run parallel ; leaving between them, 

 along the median line, a thick ridge {Fig. 15 y), which is 

 nearly as broad as the diameter of the tubes. The relations 

 of these parts, as seen in profile, may be better understood 

 by referring to the annexed wood-cut {Fig. 87) and the ac- 

 companying explanation, at the same time comparing it with 

 PI. IP. Fig. 15, which is lettered correspondingly. The thick- 

 ness of the disk as a whole is equal to about two thirds 

 of the breadth, but it thins out at the actinal end,(;'") in 

 just about the same proportion that it . narrows ; whereas at 

 the other extremity {(i'"), where it is slightly narrowed, it is 

 thickest, and also terminates • abruptly ■ directly opposite the 

 junction of the main chymiferous trunk {q) with the two par- 

 allel tulles {a a a") just mentioned. It seems appropriate 

 to compare the shape and proportions of this disk to a 

 flat-soled, broad shoe-last. The structural details of this organ 

 appear very simple, when once fully understood ; but, owing 

 to the fact that it is situated at the bottom of a deep socket 

 {j f f-) and as if plunged into the midst of other organs, Longitudinal section of the 



J. o o 7 tentacular apparatus of Pleuro- 



it seems to be quite complicated. Only imagine the socket brachia khododactvla, .4-/.1 

 to be removed or reverted, as oftentimes does happen in a great measure, and 

 the whole apparatus will , appear like a peripheric ridge, which, at one point, is 

 drawn out into a slender thread, the tentacle. The hn,se {Fig. 15 g g^ g"^) of the 

 tentacle has the form of a high, narrow ridge or keel {Fig. 15 g^ g^ and Fig. 87 /), 

 more or less plicated or distorted, according to whether the apparatus is extended or 

 retracted ; but we have never seen it projecting beyond the aperture {j^) of the 

 socket. At the basal end {g) of the keel it is as broad as the disk from which it 

 arises, but it suddenly narrows to a uniform thickness, ' which it retains to the other 



^ Fii/. 87 represents a longitudinal section of 

 Fiff. 15, PI. IP, one of the chymiferous tubes (a a") 

 being in tlie distance ; e wall of the main horizontal 

 chj-miferous trunk ; e^ opposite side to e where it 

 passes into the iuner wall (;■'") of the disc ; ff tlie 

 base of the tentacle ; J tentacular socket ; j^ aper- 

 ture of j ; j^ apex of J near to where it passes 

 into the outer wall (j3") of the disc ; j^ the inner 

 or proximal side of j where its wall passes into 

 the outer layer ('^'") of the disc ; k the tentacle ; 

 q point of junction of e and a a' ; the dotted lines 



represent the outlines of the channel which diverges, 

 at right angles, from e (see q^ Fiff. 15, PI. 11") ; 

 a the apex of the chymiferous tube ; a' entrance 

 to a ; a" base of a ; ^" outer wall of the disk ; 

 f>"' same as §" at the thickest part of the disk ; 

 7 the inner layer of the disk ; / inner layer of 

 ff ; /' the thill proximal wall of a where it passes 

 into y ; /" same as ;'" further along ; y"" the 

 thickest part of the inner layer of the disk. De- 

 signed from nature by H. J. Clark, to correct the 

 mistake alluded to p. 234, note. Magnified. 



