=Cephalopoda.=--In the Cephalopoda the concentration of ganglia attains its maximum, and may perhaps be regarded as approaching the point at which a definite brain may be said to exist. Another point of distinction is the formation of special small ganglia upon the nerve-cords in different parts of the body. In the Tetrabranchiata (_Nautilus_) the cerebral and pedal ganglia form a broad ring which surrounds the oesophagus, the former giving out the optic nerves, with their special optic ganglion, and a pair each of buccal and pharyngeal ganglia, the latter the nerves of the arms and funnel. The visceral loop is still present in the form of a separate band, which innervates the branchiae, osphradia, and viscera generally, forming a special genital ganglion in connexion with the reproductive organs. The principal ganglia of the Dibranchiata are still more concentrated, even the visceral loop being possibly united with the rest in forming an unbroken mass in which scarcely any trace of commissures can be detected. The pedal ganglion becomes separated into two portions, one of which innervates the arms, the other the funnel. Two peculiar ganglia (the stellate ganglia) supply a number of branching nerves to the mantle. [Illustration: FIG. 106.--Nervous system of _Cardium edule_ L.: _a.m_, anterior adductor muscle; _br_, branchiae; _br.n_, branchial nerve; _c.g_, _c.g_, cerebral ganglia; _c.p.c_, cerebro-pedal commissure; _c.v.c’_, cerebro-visceral commissure; _c.v.c_, cerebro-visceral commissure of mantle; _l.p_, labial palps: _m_, mouth; _p.g_, pedal ganglion; _p.m_, posterior adductor muscle; _v.g_, visceral ganglion. (After Drost, × 3.)] * * * * * =E. L. Bouvier=, Système nerveux, morphologie générale et classification des Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7), iii. 1887, pp. 1–510. =J. Brock=, Zur Neurologie der Prosobranchier: Zeit. wiss. Zool. xlviii. 1889, pp. 67–83. =O. Bütschli=, Bemerkungen über die wahrscheinliche Herleitung der Asymmetrie der Gasteropoda, etc.: Morph. Jahrb. xii. 1886, pp. 202–222. =B. Haller=, Zur Kenntniss der Muriciden. I. Anatomie des Nervensystems: Denksch. Math. Nat. Kl. Ak. Wien, xlv. 1882, pp. 87–106. „ Untersuchungen über marine Rhipidoglossen. II. Textur des„ Centralnervensystems und seiner Hüllen: Morph. Jahrb. xi. 1885, pp. 319–436. =H. Grenadier=, Abhandlungen zur vergleichenden Anatomie des Auges: Abh. Naturf. Gesell. Halle, xvi. 1884, pp. 207–256; xvii. 1886, pp. 1–64. =A. P. Henchman=, The Origin and Development of the Central Nervous System in _Limax maximus_: Bull. Mus. C. Z. Harv. xx. 1890, pp. 169–208. =V. Hensen=, Ueber das Auge einiger Cephalophoren: Zeit. wiss. Zool. xv. 1865, pp. 157–242. =C. Hilger=, Beiträge zur Kenntniss des Gasteropodenauges: Morph. Jahrb. x. 1885, pp. 352–371.