Fam. 7. Chiroteuthidae.—Head nearly as large as the body; fins terminal, tentacular arms very long, sessile arms slightly webbed, acetabula denticulated; mantle-supports consisting of cartilaginous ridges on the mantle, which fit into corresponding depressions on the funnel, gladius expanded at each end.—Atlantic Ocean.
The six dorsal arms in Histioteuthis are united by a broad web, while in Histiopsis the web only reaches half way up the arm. In Chiroteuthis the tentacular arms have scattered sessile suckers throughout their whole length, and four rows of very long pedunculate suckers on the clubs.
Fam. 8. Cranchiidae.—Head small, body rounded, barrel-shaped, fins terminal, eyes often very large, sessile arms short, tentacular arms long, thread-like.—World-wide.
Cranchia proper has the tentacular clubs finned, with eight rows of suckers, body sometimes covered with warty tubercles. Loligopsis has a very attenuated body, with fins terminally united; some species are spotted with colour, or have rows of tubercles on the ventral side. Taonius (Fig. [251]) is doubtfully distinct from Loligopsis.
Fig. 251.—Taonius hyperboreus Stp., N. Atlantic: e, e, eyes; f, f, fins; t, t, tentacular arms. (After Hoyle, × ¼.)
Order Tetrabranchiata
Cephalopoda with four branchiae and four kidneys; animal inhabiting the last chamber of an external multilocular shell; funnel consisting of two separate lobes; tentacles numerous, without suckers or hooks; no ink-sac.
The shell consists of two layers, the outer being porcellanous, and the inner, as well as the walls of the chambers or septa, nacreous. The septa vary greatly in shape. In most of the Nautiloidea they are regularly curved, as in Nautilus, or straight, as in Orthoceras, but in the Ammonoidea they are often exceedingly complex. The edge of the septum, where it unites with the shell-wall, is called the suture, and the sutural line, which is not seen until the porcellanous layer is removed, varies in shape with the septum.
