222 Dr. W. B. Carpenter on the Structure [Jan. 20, 



zontal partition separating the coeliac from the subtentacular canals, a 

 longitudinal passage is left (which may be called the genital canal), in 

 which lies what I have shown to be the generative rachis (fig. 6, gr), as 

 Prof. Semper independently discovered. Where, again, this vertical 

 partition joins the thickened layer of perisome which forms the floor 

 of the inter-tentacular groove, there is a small circular canal (overlooked 

 by Miiller and every other anatomist), which is the real tentacular canal 

 (tc, figs. 6, 8, 9), its continuity with the interior of the tentacles being 

 very distinctly visible both in transverse and in longitudinal sections. 

 The latter show a single oval orifice (oo, fig. 9) opening out of the 

 tentacular canal for each group of three tentacles. 



The perisome covering the oral surface of each arm exhibits a con- 

 tinuation of the brachial furrow formed by the divarication of the radial 

 furrow of the oral disk, each side of this furrow being bounded, as in 

 the disk, by successive groups of tentacles, and by an elevated fold of 

 the perisome, scolloped at its edge so as to form a row of minute valvules, 

 like those of the oral disk, beyond which, when the tentacles are extended, 

 they project considerably, but within which they may be withdrawn, so 

 that the two folds may close down, the valvules meeting in a sinuous line 

 so as completely to cover the furrow. — The floor of the furrow is formed 

 by the peculiar layer which, as already stated, I regard as a columnar 

 epithelium ; its free surface I believe to be clothed with minute cilia, the 

 direction of whose movement is from the periphery towards the centre ; 

 there are,- however, no cilia on the tentacles. These points, correctly 

 stated by M. Edmund Perrier, I had worked out soon after the publica- 

 tion of my previous Paper. 



Pinnules. — Every segment of the arms bears a pinnule ; and the pin- 

 nules are disposed alternately along the two sides of each arm. Their 

 calcareous segments are united by ligaments alone ; but each centrum (a, 

 figs. 8, 9) is perforated as in the arm, and contains a branch (5) of its 

 axial cord. On the oral face of the succession of calcareous segments 

 there lies an extension (cc, figs. 8, 9) of the coeliac canal of the arm ; and 

 immediately above this, in the barren pinnules, lies a single subtentacular 

 canal coming off from the lateral division of the double canal of the arm 

 which belongs to the side that bears the pinnule. In the thickness of the 

 horizontal partition between these two canals there is a passage left by a 

 separation of their walls, in which lies a lateral branch of the generative 

 rachis. The true tentacular canal (tc, figs. 8, 9) lies, as in the arms, along 

 the oral face of the subtentacular canal. The first pinnules of each arm, 

 however, are differentiated from the rest by the absence of tentacles, and 

 likewise by the greater number and closer setting of the peculiar sacculi 

 (ss, figs. 6, 8) contained in the perisomal folds which bound the radial 

 furrow. 



When the tumefaction of a pinnule shows either the testis or the ovary 

 to be developed in it, this organ is found to lie between the coeliac and 



