THE TETRABRANCHIATA. 



457 



are set a great number of transverse plates, in such a manner 

 that the axis of the stem passes through the centre of the 

 plates. The anterior and lateral regions of the hood are 

 completed by two narrower processes, each of which contains a 

 similar tentacle, and the lateral portions of the sheath are 

 formed by sixteen or seventeen smaller tentaculiferous pro- 

 cesses the surfaces of which are more or less distinctly an- 

 nulated When the sheath is opened out, there is seen to be 

 attached to its inner surface, on each side, close to the reen- 

 tering angle between it and the lip which surrounds the beak 

 and along the line of junction of the lateral part of the sheath 

 with the isthmus, a thin, free, quadrate lobe, which carries 

 twelve tentacles. The isthmus joins the posterior edo-es of 

 these outer tentaculiferous lobes, as well as those of the two 

 halves of the sheath, and it exhibits on its anterior, or inner 

 surface a broad area beset with delicate, close-set, curved 

 laminae. Two other similar, but much thicker, inner tenta- 

 culiferous lobes, which also carry twelve tentacles, lie be- 

 tween these and the lip. They are quite free from the outer 

 tentaculiferous lobes, and unite with the sheath only above 

 and behind Like the halves of the sheath, these two lobes 

 are united behind by a thick isthmus, the surface of which 

 presents a number of parallel longitudinal lamina? The 

 beak which is hidden by the sheath and the lobes, is sur- 

 rounded by the thm circular lip already mentioned, the free 

 margin of which is papillose. Besides these, there is a short, 

 conical tentaculiferous process above the pedunculate eye, and 

 another below it. In the male, the internal tentaculiferous 

 lobes are wanting, and the outer tentaculiferous lobes are 

 divided into two portions, an anterior which bears eiffht and 

 a posterior with four, tentacula. On the left side, the'four 

 tentacles of the posterior division have undergone much mod- 

 tnW- 0n ' w\T conv ^d into a peculiar organ termed the 

 spadix, which bears a discoidal follicular gland upon its outer 

 surface. .There is thus a kind of hectocotvlization in the 

 letrabranchiata. 



fuT^ 116 f argi i nS °i the UIlited e P ] 'P° dia are not united into a 

 tubular funnel. They constitute a muscular membrane, nar- 

 row on the anterior face of the body, but becoming wide and 

 hind m S a manner that its P° steri or edges overlap, be- 



The mantle has a broad anterior fold, which covers the 

 anterior convexity of the shell, and the region which it thus 

 inverts is black. The pallia! chamber does not ext end for 

 JO 



