SQUID, the popular name given to a large number of cephalo pod molluscs which include (e.g.) the common squids of the Atlan tic (Loligo, Ommastrephes), the giant squids (Architeuthis) and the fire squids (Lycoteuthis, Abralia). They are exclusively marine animals of world-wide distribution and form an important part of the population of marine swimming and floating animals.
A typical squid, e.g., Loligo vulgaris, the common squid of the north-east Atlantic and Mediterranean, has an elongate slender body (visceral sac) edged by triangular fins, a short square head provided with well-developed eyes and ten arms. On the under surface of the latter are arranged rows of suckers which are strengthened by tough horny rings. Two of the arms (tentacles) are longer and more mobile than the other eight (sessile arms) and the suckers are concentrated at the extremity as a "hand." The long tentacles are used for capturing prey, the sessile arms for transferring the latter to the mouth and grasping it while it is being crunched by the horny jaws which are situated around the mouth in the centre of the circlet of arms.
Situated within the muscular tissues of the "mantle" is a long horny structure, the "pen" or gladius. This is the rudiment of a shell and consists actually of a secondary growth (the proostra cum) which has replaced the true shell-rudiment. In the genus Ommastrephes there is a vestige of the original shell.
The majority of the squids are active, highly mobile animals of aggressive habits and are probably all carnivorous.
Originally the squids were distributed among the two "tribes," Egopsida and Myopsida, of the ten-armed Cephalopoda (Deca poda). Recently, however, Naef and other students have proposed a more suitable classification of the Decapoda according to which the squids are all placed in a single sub-order, the Teuthoidea.
These forms are divisible into two groups originally called by C. Chun the Oegopsida libera and Oegopsida consuta. The first include the large, active and freely swimming forms (Architeuthis, Loligo, Ommastrephes, etc.). The second comprise a smaller as semblage of curious and highly modified forms which are less common than the Libera and are very largely planktonic. The
distinction between these two groups is based on the mode of at tachment of the ventral edge of the mantle to the head. In the Libera this attachment is effected by a stud and socket junction (see CEPHALOPODA) between the head and the edge of the mantle and can be freed from the head when necessary. In the Consuta the ventral edge of the mantle is permanently fixed to the head.
The Libera are represented by about 65 genera. Among them Architeuthis is the most striking on account of its size. The genus includes the largest living invertebrate animals, Architeuthis prin ceps of the north Atlantic attaining a maximum record length of 52 ft. (including the outstretched tentacles). Sthenoteuthis, a group of smaller forms, which attain a size of over seven feet, may be also reckoned "giant squids." Another interesting group are the phosphorescent squids, Nematolampas, Thaumatolampas (Lycoteuthis) and Abralia, which bear light-organs in certain regions of the body (on the mantle, arms, inside the mantle-cavity and around the eyes). Cer tain of these animals when seen alive are very beautiful creatures.
Ommastrephes bartrami, the so-called "flying squid," is found throughout tropical and temperate seas. Its flight is, strictly speak ing, a series of leaps across the surface of the sea which are often strong enough to land it on the deck of a ship. This feat is no doubt rendered easier in high seas.
The Consuta are mainly small animals. Their fins are largely reduced and the mantle is swollen and tun-like in Cranchia, Bathothauma, etc. In the latter, and in Sandalops, and Corynom ma the arms are much reduced and the eyes are situated on stalks. Unlike the larger swimming squids these animals probably float largely at the mercy of the currents and are thus to be reckoned as "plankton." (See CEPHALOPODA.) See G. Pfeffer, Die Cephalopoden der Plankton-Expedition (1912) ; C. Chun, Wiss. Ergebn. d. Deutsche Tiefsee Expedn. Bd. xviii. (1910).
(G. C. R.)