The first form commencing with Area leads through Mytilus and Pecten to Ostreal where it is degenerate, 'the second commences with Unto and leads to Venus and Scrobicularia' (Grobben). Venus has also rudiments of the gland on the auricles, probably the more primitive form. Concretions are usually to be found in the epithelial cells, which are sometimes ciliated as in Area and Mytilus, but not in Unio, etc. The first form resembles that found in Cephalopoda 1.

1 For the terms used in this description see the note, p. 452, ante; and for a full account, Patten, op. cit.

2 This is the usual statement; but in Anodonta the nerve is said to be derived from the cerebro-pedal connective. See p. 138.

Each ctenidium or gill consists of a longitudinal axis, united for the greater part of its length to the walls of the body but free at its posterior extremity. It contains an afferent and efferent blood-channel, the former bringing blood from the nephridia through which it passes from the median infra-cardiac sinus, the latter conveying it back to the auricles. The ctenidial axis bears two rows of gill-filaments. These are simple and lamellate in Nucula and Yoldia (Areaeea), but are usually more or less tubular, and recurved or bent upon themselves, the outer row externally, the inner row internally. And then either the recurved portion is united to the direct portion by solid interlamellar junctions, and also laterally by interfilamentar ciliary junctions (Area Pectunculus, Mytilus), or by the development of solid interfilamentar junctions, the lattice-like structure of the branchia seen in most Lamellibranchiata is attained. The recurved portions of the outer row of filaments generally unite with the mantle; of the inner row anteriorly with the body, posteriorly with the corresponding row of the opposite side, but they may be either free or united in the region between.

With the development of interlamellar and inter-filamentar junctions the blood-channels cease to run for the most part in the filaments but follow the course of the junctions. At the same time the branchial cavity is divided into an infra- and a supra-branchial cavity by the union of the recurved portions of the inner filaments as described above, cf. p. 130, ante. The anus lies at the posterior extremity of the supra-branchial cavity, which is continued on into the exhalent siphon when present, the cavity of the inhalent passing into the infra-branchial chamber. The nephridia are always paired, and bent or folded upon themselves. The ducts open externally by well-marked pores, usually placed as in Anodonta, (p. 134, cf. Pl. vi.), and the two ducts usually communicate one with another near the external opening. The glandular portion of each organ generally communicates with the pericardium, but the ciliary currents are said to set outwards as a rule. The Oyster differs from other Lamelli-brancJiiata in the structure of the nephridia, see p. 291. The genitalia are paired branched glands terminating in round or cylindrical caeca in which the genital products are formed.

The branches are intermingled with the branches of the liver, but may be partially (Anomia) or wholly contained in the mantle (Mytilns). The ducts open as a rule near the neph-ridial openings, or into a groove common to the two sets of apertures (Ostrea, Arca, Pinna, etc.), which may be converted (?) into a canal with a small opening (Pecten, Lima, Spondylus). The sexes are separate with few exceptions such as Ostrea edulis, Cardium serratum, in which male and female products are developed in the same caeca but at different times; or Pecten, Aspergillum, etc, in which the male and female parts of the gland are more or less separate, but both genital products ripen at the same time and the animals may therefore be self-impregnating. Cyclas and Pisidium are also hermaphrodite, but it is not certain to which group of the two described they belong. The spermatozoa find their way to the ova either in the water, or, what appears to be generally the case, in the mantle cavity, or in that of the outer gill. In Kellia, Galeomma and Montacuta bidentata the ova develope within the ovary, but it is not certain whether the animals are hermaphrodite and self-impregnating, or the spermatozoa pass up the genital duct.

The ovum is enveloped in a membrane, which is sometimes (Anodon, Unio) a vitelline membrane, or is formed by the superficial hardening of an albuminous layer (Scrobi-cidarid). It is attached to the wall of the ovary by a protoplasmic stalk. Hence the membrane is incomplete at this spot, which forms the micropyle when the ovum is detached. The ovum always remains connected with the envelope at the micropyle, but is separated elsewhere by an albuminous layer. Segmentation is unequal. The gastrula is either invaginate or formed by overgrowth. The velum of the Veliger is a circular ridge, and from the centre of the velar area arises a long flagellum. This is wanting in many marine forms, e. g. Ostrea, and the velum itself is in some fresh-water forms reduced (Anodon, Unio, Cyclas) or absent (Pisidium). In the case of Cyclas and Pisidium development takes place in a special brood-pouch which is connected with the root of the inner gill-lamella on each side. The young bivalve appears to be nourished by a clear fluid secreted by the walls of this pouch, as are the young of Anodon (and Unio?) by a secretion from the gills.

1 Grobben, Z. A. ix. 1886, p. 369.

Pholas is phosphorescent. The luminous matter is secreted by cells disposed in a band along the anterior edge of the mantle, in two spots at the base of the inhalent siphon and two bands along the same tube.

The Lamellibranchiata are mostly marine. They are either fixed or free and they occur associated in numbers. They feed on minute organisms suspended in the currents of water caused by the motion of the cilia, clothing the mantle surfaces, the edges of the gill-lamellae, and the labial tentacles. The living genera Arca and Mytilus appear in the lower Silurian, and there are large numbers of fossil forms known.

The class may be divided as follows:

1. Isomya

Anterior and posterior adductor muscles of approximately equal size.

(I) Integripallia

Marginal attachment of the mantle to the shell not inflected to form a sinus; siphons not developed in some, present in most, e. g. Arcacea, Trigoniacea, Unionacea.

(II) Sinupallia

Marginal attachment of the mantle to the shell inflected so as to form a sinus into which the pallial siphons can be withdrawn; siphons always present and large: e. g. Myacea, Pholadacea.

2. Heteromya

Anterior adductor (pallial adductor) much smaller than the posterior adductor (pedal adductor); siphons rarely present: e. g. Mytilacea.

3. Monomya

Anterior adductor absent in the adult; siphons never developed: e. g. Ostreacea.

For literature, see pp. 127, 131, 133, 138.

Anomia, de Lacaze Duthiers, A. Sc. N. (4), ii. 1854. Aspergillum, Id. A. Z. Expt. (2), i. 1883. Mytilus, Sabatier, A. Sc. N. (6), v. 1877.

Eye, Patten, Mitth. Zool. Stat. Naples, vi. 1886, with lit. quoted.