Fam. 1. Cirrhoteuthidae.—Body with two prominent fins; arms in great part united by a web; one row of small suckers, with cirrhi on each side.—Atlantic and Pacific Oceans, deep water (Fig. [241]).
Fam. 2. Amphitretidae.—Body gelatinous, mantle fused with the funnel in the median line, forming two openings into the branchial cavity; arms with one row of suckers; umbrella extending more than two-thirds up the arms.—South Pacific (Fig. [242]).
The two pocket-like openings into the branchial cavity are unique among Cephalopoda (Hoyle).
Fig. 242.—Amphitretus pelagicus Hoyle, off Kermadec Is.: e, eyes; f, funnel; p, right mantle-pocket. (After Hoyle.)
Fam. 3. Argonautidae.—Female furnished with a symmetrical, unilocular shell, spiral in one plane, secreted by thin terminal expansions (the vela) of the two dorsal arms, no attachment muscle; suckers in two rows, pedunculate; male very small, without veligerous arms or shell.—All warm seas (Fig. [243]). Pliocene——.
The shell consists of three layers, the two external being prismatic, the middle fibrous. Its secretion by the arms and not by the mantle edge is unique, and shows that it is not homologous with the ordinary molluscan shell.
Fig. 243.—Argonauta argo L., the position assumed by a specimen kept in captivity, the arrow showing the direction of movement: f, funnel; m, mouth, with jaws projecting; sh, shell, with arms as seen through it; wa, webbed arm clasping shell. (After Lacaze-Duthiers.)
The great controversy on the Argonauta, which once raged with so much fierceness, is now matter of ancient history. It seems scarcely credible that between fifty and sixty years ago, two of the leading zoologists of the day, Mr. Gray and M. de Blainville maintained that the animal which inhabits the Argonaut shell is a parasite, without any means of depositing or forming a shell of its own, but which possesses itself of the Argonaut shell, either by expelling or succeeding the original inhabitant, a supposed nucleo-branchiate (Heteropod) mollusc akin to Carinaria. The final blow to this strange hypothesis—which was urged by the most ingenious series of arguments—was given by Professor Owen, who in 1839 brought before the Zoological Society of London the admirable observations of Madame Jeannette Power, who made a continuous study of a number of specimens of Argonauta in her vivarium at Messina. The result of these observations tended to show that the young Argonauta when first excluded from the egg is naked, but that in ten or twelve days the shell begins to form, that the principal agents in the deposition of shell are the two velated or web-like arms; and that portions of the shell, if broken away, are repaired by a deposition of calcareous matter.[397]