The heart is placed sometimes in front of, sometimes behind, the base of the ctenidia, and then the auricle lies either anteriorly or posteriorly in the body, with reference to the ventricle. Hence the terms prosobranchiate and opisthobranchiate. The blood consists of a colourless plasma with amoeboid corpuscles. In Planorbis (Pulmonata) the plasma is tinged with haemoglobin, and it contains haemocyanin in various Streptoneura, e.g. Haliotis, Fissurella, Turbo, Murex, and in Helix among Pulmonata2.

1 In Turbo the pericardium is perforated, but not the ventricle; in Neritina neither. In these two genera and in Nerita the intestine is usually said to pass through the ventricle. Cf. Landsberg, Z. A. v. 1882.

2 A pericardial gland in the form of lobes upon the auricle occurs in Fissurella, Parmophorus, Haliotis, Turbo, and Trochus, according to Grobben, Z. A. ix. 1886, p. 371.

The respiratory organs are represented either by ctenidia, or by the surface of the branchial cavity formed by the mantle, or else by secondarily formed processes of the body wall. Processes of every kind are absent, and the general surface of the body is respiratory in certain Non-Palliata - the Haplornorpha, e.g. Phyllirhoe. Two ctenidia are present only in the Haliotidae, and Fissarellidae among Zygobranchia. In all other Anisopleura there is but one. But in some Azygobranchia the osphradium is large, thrown into folds, and is generally taken for a second but reduced cteni-dium (parabranchia). The single persistent ctenidium is on the left side of the anus, and therefore represents the original right ctenidium. The Limpets (Patellidae) among Zygobranchia have the two ctenidia quite rudimentary, forming the capito-pedal organs of Lankester. The single ctenidium of palliate Opisthobranchia is also lost in the Phyllidiobranchia. In both cases vascular lamellae or pallial gills are developed in the mantle furrow, forming a complete series in the former of the two groups (Patellidae), a right and left row in the latter.

A circle of pinnate folds surrounds the anus in certain Opisthobranchia Non-Palliata, i.e. the Pygobranchia, e.g. Doris. It is probable that they are secondary processes, not true ctenidia, and in certain members of the same sub-order, as well as in the sub-order Ceratonota of Non-Palliata, cylindrical hollow processes of the body walls - the cerata - are developed, more or fewer in number, and extending in lateral rows on each side of the body. Into these cerata liver caeca may extend (supra). In one group of Azygobranchia - the Pneumochlamyda, e.g. Cyclostoma - and in the Pulmonata, the roof of the branchial cavity formed by the mantle has taken on a respiratory function, and is traversed by a network of vascular sinuses. In the genus Ampullaria (Azygobranchia Holochlamydd) from the fresh-waters of tropical America, Africa, and East Indies, one side of the roof of the branchial cavity is respiratory, but a ctenidium is present as well.

Two nephridia are found in some Zygobranchia (Fissurella, Patella), but not in others (Haliotis). One of the two - the left, i.e. the primitive right nephridium - is rudimentary. It appears to be the one that is aborted in other Anisopleura, and not the right (primitive left), as is generally maintained. In Haliotis, and still more in Turbo, where the organ is on the right side of the body, its duct crosses the intestine and opens on its left side. In other instances the organ itself appears to undergo a greater or less change of position1. The nephridium consists of a urinary chamber opening externally, of secreting and usually branched acini, the calibre of which is variable, and a canal which connects the pericardium and the urinary chamber. T'his chamber is primitively small, e.g. Fissurella, and lined by a ciliated secretory epithelium, but it is generally large and its epithelium non-secretory. In Non-Palliata it is a wide sac extending antero-posteriorly. In this group the secreting acini may be lost, e.g. Phyllirhoe, but they are as a rule branched tubes scattered along the edges of the chamber, e.g. Bornella. The acini are also branched in other instances, and often grouped into two lobes, anterior and posterior, of which the former may be lost (Pulmonata). They are either large or small; either free, e.g. Fissurella, or united by connective tissue; and are lined by a ciliated secretory epithelium, with either similar or dissimilar cells.

In the latter case the two kinds of cells may be found, either side by side or confined to separate lobes. The differences relate to form and character of the excretory products. The urinary chamber either opens direct by a simple aperture, e.g. Buccinum, or by a papilla, or a long duct, e.g. some Pulmonata. The opening lies within the branchial cavity, either near the base of the gill, or, when a duct is present, hear the anus. Its position seems to be somewhat variable.

1 The point is one which requires further research. It is possible that the Anisopleura do not all agree. See Haller, M. J. xi. 1886, p. 26. In Paludina the nephridial duct is to the right of the rectum; between it and the vagina.

The sexes are separate in the Streptoneura, but the Euthyneura are hermaphrodite, the male and female products ripening at different times (successive hermaphroditism). When a Gastropod is hermaphrodite, the sperm and the ova either develope in the same follicle of the gland, e.g. Pulmonata, or the sperm in a central capsule, into which open a number of smaller capsules containing ova, as in many ceratohotous Non-Palliata; or finally, as in Elysia (Non-Palliata Haplomorpha), there are two separate glands. The genital gland consists of simple or branched acini, and usually lies in or near the liver. It is often lobed or compact in form, usually single but sometimes double, e.g. hermaphrodite gland of Phyllirhoe, testis of Paludina. The efferent ducts are ciliated in some cases1 When the sexes are separate the vas deferens is usually a simple canal devoid of any accessory glands, e.g. Natantia. It either opens on the left side of the anus, and a ciliated furrow either leads to the base of the penis, e.g. Murcx, Natantia, or is continued along it, e.g. Strombus, etc.; or it is continued onwards as a canal, and opens either at the base or at the apex of the penis, e.g. Buccinum. The penis, placed on the right side near the head, is rarely hollow and invaginable, usually solid and muscular.