The only genus, Idiosepion, with a single species (I. pygmaeum Stp.) is from the Indian Ocean, and is the smallest known Cephalopod, measuring only about 15 mm. in length.
Fam. 4. Loliginidae.—Body rather long, fins varying in size, tentacular arms partially retractile, gladius as long as the back, pointed in front, shaft keeled on the ventral side.—World-wide.
Loligo proper has a pointed body with triangular posterior fins united behind; sessile arms with two rows of acetabula, tentacular arms with four; fourth left arm hectocotylised at the tip; funnel attached to the head. Other genera are Loliguncula, Sepioteuthis, and Loliolus. Belemnosepia, Beloteuthis, Leptoteuthis, and Phylloteuthis are fossil genera only, differing in the shape of the gladius.
(b) Oigopsidae: cornea more or less open; species pelagic.
Fam. 5. Ommastrephidae.—Body cylindrical, fins generally terminal, united together, regularly rhomboidal, sessile arms with varying number of rows of acetabula, mantle connexions elaborate; gladius horny, narrow lanceolate, with a hollow cone at the posterior end.—World-wide.
Fig. 250.—Architeuthis princeps, Verr., E. America: f, Right fin; fu, funnel; f.c, fixing cushions and acetabula on the tentacular arms (t, t). (After Verrill, × 1/60.)
Ommastrephes proper has a natatory web on the sessile arms; the wrist of each club has a series of acetabula with corresponding cushions on the other wrist. In Thysanoteuthis (often made a separate family) the sessile arms have two rows of cirrhi, with lateral expansions of the skin; fins as long as the body. In Architeuthis, to which belong the largest Cephalopoda known, the fins together are shaped like a broad arrow-head; acetabula of sessile arms strongly denticulate; tentacular arms very long, with equidistant pairs of acetabula and fixing cushions throughout their entire length, and a group of the same at the base of the club. The acetabula and cushions correspond on the opposing tentacles, and enable them to pull together. Other genera are Dosidicus, Todarodes, Illex, Bathyteuthis and Mastigoteuthis.
Fam. 6. Onychoteuthidae.—Body cylindrical, fins terminal or lateral, mantle-locking apparatus elaborate, tentacular arms very long, sessile or tentacular arms furnished with retractile hooks, gladius lanceolate, with a terminal cone.—World-wide.
The prehensile apparatus of Cephalopoda reaches its maximum of power and singularity in this family. In Onychia, Onychoteuthis and Ancistroteuthis, the sessile arms have acetabula only, in Gonatus and Abralia they have hooks as well, while in Verania, Ancistrochirus and Enoploteuthis, the sessile arms have hooks only. The number of rows of hooks or acetabula varies with the different genera.