The Cephalopoda are marine, some littoral like Octopus, others pelagic like Spirula, Sepia, etc. All are carnivorous, and some, e.g. Architeuthis, attain a gigantic size. The oldest Cephalopoda belong to the Nautiloidea, and appear in Cambrian strata. Of existing genera, Loligo is found in the Lias, Ommastrephes and Sepia in the Oxford Clay.

The class Cephalopoda is divisible into two orders.

I. Tetrabranchiata

Shell external; straight or coiled; chambered; the last chamber inhabited by the animal, the remainder filled with gas. A membranous tube or siphuncle traverses the chambers. Mostly extinct: one genus only, Nautilus, living, with the following characters. The part of the foot surrounding the mouth forms lobes which bear sheathed tentacles; the sipho is formed by two free folds. The eyes are simple open sacs; there is a pair of osphradial papillae. The viscero-pericardial sac opens externally by two apertures. There are two pairs of gills and nephridial sacs; two genital ducts, the left rudimentary in both sexes. No auricles, no branchial hearts nor ink-bag.

(1) Nautiloidea

Shell straight or bent; coiled, and then snail-like, or disc-like; aperture simple, sometimes much contracted; ventral side indicated by a notch. Sutures mostly simple, or waved; rarely denticulated. Septa concave forwards. Sipho often contracted by internal deposits; siphuncular collars generally directed backwards. Protoconch conical, with a cicatrix.

(2) Ammonoidea

Shell usually coiled and disc-like; rarely snail-like or straight; aperture simple, or with lateral and ventral processes. Sutures waved, and often denticulated. Sipho always marginal; never contracted by internal deposits. Protoconch globular or ovate. Aptychus or Anaptychus generally present (p. 456, note).

The form of the protoconch, mode of commencement of the sipho. absence of

The ventral lobe of the buccal membrane in the female Sepia contains two pouches; in Loligo one. After congress spermatophores are found on the buccal membrane, and after a time spermatozoa in the pouches in question, which they reach by their own movements. The female Loligo has been observed holding the newly laid ova in front of the mouth. Viailleton, C. R. 101. 1885. See ante, note, p. 461. cicatrix, presence of a pro-sipho, and character of the Aptychus, make it possible that the Ammonoidea belong to the Dibranchiata.

II. Dibranchiata

Shell chambered and partially external (Spirula); or chambered with a guard, and internal (Belemnitidae); plate-like and internal (Decapoda); or absent (Octopoda). An external calcareous but not chambered shell in the female Argon-auta Argo (Octopoda) formed by a pair of arms. Foot surrounding mouth produced into sucker-bearing arms. Lobes of siphon fused. Eyes large; retinal chamber closed by a lens; no osphradia. A single pair of ctenidia; a pair of auricles and branchial hearts; and of nephridia into which the viscero-pericardial sac opens. Two vasa deferentia in Eledone moschata; two oviducts in Ommastrephes, and in Octopoda; otherwise genital duct unpaired. An ink-bag.

(1) Decapoda

Ten arms; one pair much elongated; suckers pedunculate, and strengthened by a horny ring; lateral body fins. Nidamental glands usually present. Belemnitidae extinct; Mesozoic (Trias to Chalk); ? Tertiary. Oegopsidae, cornea perforate, e. g. Ommastrephes; Myopsidae, cornea closed, e. g. Sepia, Loligo. Spirulidae, a chambered skull; Spirula Peronii, warmer seas.

(2) Octopoda

Eight arms, with a more or less developed inter-brachial membrane. Eyes with a sphincter-like lid. Cirrhoteuthidaei. Philonexidae, e. g. Ocy-thoe, Argonauta; Octopidae, e.g. Octopus, Eledone.

Cephalopoda, Keferstein, Bronn's, Klass. und Ordn. des Thierreichs, iii. 2, 1862-66; Hoyle (Systematic and distribution), Challenger Reports, xvi. 1886.

General account of Nautilus, Ray Lankester, Encyclopaedia Brit. (ed. ix.), xvi. p. 670 et seqq. Osphradium and genital ducts, Id. and Bourne, Q. J. M. xxiii. 1883. Male and female; differences, Bourne, Nature, xxviii. 1883.

Dibranchiata. Skin', chromatophores; suckers, Girod, A. Z. Expt. (2), i. 1883; ii. 1884. Chro7iiatophores, Blanchard, C. R. xcvi. 1883 (A. N. H. (5)xi.); Physiology, Krukenberg, Vergleich. Physiol. Studien, i. 1. 1880. Suckers, Niemiec, 'Les ventouses,' etc, Recueil Zool. Suisse, ii. 1885. Ctenidia, development and structure, Joubin, A. Z. Expt. (2), iii. 1885. Nervous system of Ommastrephes, Hancock, A. N. H. (2), x. 1852. Eye, Carriere, 'Seh-Organe der Thiere,' Leipzig, 1885; Grenacher, Abhandl. Nat. Gesellsch. Halle,'xvi. 1884; cf. note, p. 452, ante; of Procalistes, Ray Lankester, Q. J. M. xxiv. 1884. Digestive organs (histology), Li von, Journal de l'Anat. et Physiol, xvii. 1881. Pancreas, Vigelius, Z. A. iv. 1884; Biol. Centralbl. ii. 1882-83 \ Physiology, Bourquelot, A, Z. Expt. (2), iii. 1885. Ink-bag, Girod, A. Z. Expt. x. 1882. Excretory organs and pericardial glands, Grobben, Arb. Zool. Inst. Wien, v. 1884; Vigelius, Archiv. Nederland. f. Zoologie, v. 1879-82; Solger, Z. A. iv. 1881. Sex organs, Brock, Z. W. Z. xxxii. 1879; Id. Males of Sepioloidea, Z. W. Z. xl. 1884.

Various points relating to anatomy, etc, Brock, 'Phytogeny,' etc, M. J. vi. 1880; Id. Z. W. Z. xxxvi. 1882. Thysanoteuthis, Vigelius, Mitth. Zool. Stat. Naples, ii. 1881.

Cartilage of Cephalopoda, Fiirbringer, M. J. iii. 1877.

Homology of siphon and arms, Brooks, American Journal of Science and Art, xx. 1880.

Spirula, Owen, A. N. H. (5), iii. 1879; P. Z. S. 1880.

Gigantic Cuttle-fish, etc, Verrill, U. S. Commission of Fish and Fisheries, Report for 1879, Part vii. Washington, 1882.

Fossil Cephalopoda, Zittcl, Handhuch dcr Palaeontologie, AUh. I. ii. Th. 3. 1884; Genera of Fossil Cephalopoda, Hyatt, Proc. Boston Soc. Nat. Hist. xxii. 1882-83. Development of shell in do,, Bianco, Palaeontogr.iphioa (N.S.), 26 and 27, 1879-81. Relation of Ammonites, Munier-Chalmas, etc. C. R. 77, 1873. Ammonites, composition of shell, Suess, SB. Akad. Wien, lxi. Abth. i. 1870.