Fam. 4. Philonexidae.—Mantle supported by two ridges placed on the funnel; large ‘aquiferous’ pores (supposed to introduce water into the tissues) near the head or funnel; suckers in two rows, pedunculate.—Atlantic and Mediterranean.
Genera: Ocythoe, arms of unequal size, no intervening membrane, third arm on the right hectocotylised (see Fig. [51], p. 138), two aquiferous pores at the base of the siphon; male very small; Tremoctopus, two aquiferous pores between the eyes, two on the ventral side of the head.
Fam. 5. Alloposidae.—Mantle edge united to the head by three commissures; arms extensively webbed, acetabula sessile. Hectocotylised arm developed in a cavity in front of the right eye.—N. Atlantic.
Fam. 6. Octopodidae.—Head very large, arms elongated, similar, more or less webbed, acetabula usually in two rows, sessile; mantle supported by fleshy bands, no cephalic aquiferous pores.
In Octopus proper the web is usually confined to the lower part of the arms; Fischer separates off as Pteroctopus a form in which it reaches almost to their extremity. The third right arm (Fig. [52], p. 140) is hectocotylised, the modified extremity being, according to Hoyle, sometimes minute, sometimes spoon-shaped, with a tendency to transverse ridges, rarely slender and very long. The relative length of the pairs of arms varies in different species. Two cartilaginous stylets, imbedded in the dorsal mantle, are said by Owen to represent the shell.
Other genera; Pinnoctopus, body furnished with broad lateral wings which meet at the posterior end; Cistopus, a large web prolonged along the sides of the arms, fitted with oval aquiferous pouches, with pores at their base, between each pair of arms; Eledone (Fig. [244]), one row of acetabula; Tritaxeopus, Iapetella.
Sub-order II.—Decapoda.—Body oblong, mouth surrounded by four pairs of sessile and one pair of tentacular arms, the latter terminated by a ‘club’; acetabula pedunculate and furnished with a corneous margin; mantle margin locked to the base of the funnel by a cartilaginous apparatus; head and anterior part of body furnished with aquiferous pores; fins present; mandibles corneous; oviduct single, large nidamental glands in the female; shell internal.
Fig. 244.—Eledone Aldrovandi Delle Chiaje, Naples, from ventral side, × ½.
The tentacular arms, which are the principal external feature of the Decapoda, are not derived from the same muscular ring as the sessile arms, but arise from the cephalic cartilage, and emerge between the third and fourth arm on each side. In Sepia they can be entirely retracted into a kind of pocket behind the eyes, while in Loligo they are simply folded over one another. In Chiroteuthis the arms are six times as long as the body, and the clubs have four rows of denticulate suckers.