Fig. 239.—‘Club’ of Loligo vulgaris L., showing the crowded pedunculate acetabula, × ½.
Fig. 240.—One of the suckers of Architeuthis dux Stp., showing the denticulate margin and corneous ring; p, peduncle.
In many Octopoda the arms are connected by a web (the umbrella), which sometimes extends up the greater part of the arms (Cirrhoteuthis, some Eledone), at others occurs only at the base. The use of the umbrella is perhaps to assist in locomotion, by alternate contraction and expansion.
A cartilaginous skeleton is well developed, especially in the Decapoda. In Sepia a cephalic cartilage forms a complete ring round the oesophagus, the eyes being situated in lateral prolongations of the same. In front of the cephalic cartilage occurs a piece like an inverted
, which supports the base of the anterior arms. The Decapoda have also a ‘nuchal’ cartilage, connecting the head with the anterior dorsal portion of the mantle, while cartilaginous knobs on the ventral mantle button into corresponding sockets on the funnel.
Sub-order I.—Octopoda.—Body round or bag-like, generally without fins, arms eight, suckers fleshy, usually sessile, oviducts paired, no nidamental glands, shell absent.
Fig. 241.—Cirrhoteuthis magna Hoyle, S. Atlantic. Two of the left arms and their web have been removed: f, funnel; fi, fi, fins; m, mouth. (After Hoyle, × 1/12.)