Coelomate Metazoa, which are primitively bilaterally symmetrical: with a soft integument, generally ciliated and richly supplied with glands: with an in tegumental fold forming a mantle or pallium, and a ventral muscular thickening of the body walls known as the foot. There is usually an external shell borne upon and secreted by the mantle. Vascular processes of the body wall form gills or ctenidia adapted to aquatic respiration. The nervous system consists typically of a pair of cerebral ganglia united by connectives to a pair of pedal and a pair of pleural ganglia, the latter continuous with a ganglionated visceral loop, of greater or less extent. There is a pair of otolithic vesicles or otocysts; a heart, and a partly lacunar blood-system; one or two renal sacs or nephridia. A large gland or liver generally opens into the alimentary canal. There is a characteristic larval form or Veliger.

Bilateral symmetry is retained in Cephalopoda, Scaphopoda, Lamel-libranchiata, and some few Gastropoda. But in the majority of the last-named class there is a marked external and internal asymmetry, the latter shown by the position of certain of the organs (heart, termination of the intestine) and the loss of others (a nephridium a gill); the former by the disposition of the dorsal part of the body or visceral dome and the mantle fold, as well as by the shape of the shell. In some cases, however, a secondary external symmetry has become established as in some Opistho-branchia and Pulmonata. The Pteropoda, though as a rule externally symmetrical, are internally asymmetrical. The mantle fold is disposed in a characteristic manner in the different classes. It is never absent in the embryo, though sometimes lost in the adult of some Pteropoda and Gastropoda. It is a continuous fold except in Lamellibranchiata, where it is formed by independent right and left folds. The same is the case in the Scaphopoda - but here the mantle folds unite or concresce ventrally as they do in some Lamellibranchiata. The space inclosed between the mantle fold and the body is the mantle- or sub-pallial cavity.

The foot assumes various shapes equally characteristic of the various classes: it is rarely completely aborted as in certain Gastropoda (Opisthobranchia Haplomorpha) and a few Lamellibranchiata (Ostreidae). It extends normally along the ventral surface below and behind the head to the posterior limit of the body. It varies in size and shape, and may be simple (Lamellibranchiata, many Gastropoda), or divided transversely into segments, the pro-, meso-, and meta-podium (some Gastropoda). In the Pteropoda the median part of the foot is rudimentary: but there are two large lateral swimming lobes, homologous probably with the epipodial lobes of the Gastropoda, and, like them, supplied by nerves from the pedal ganglion. The foot is still more modified in Cephalopoda. One portion of it, the fore-foot, as proved by the nerve-supply from the pedal ganglia, has grown round the head, and is produced into lobes or arms characteristic of the class. It may correspond with the lateral lobes of the foot in Pteropoda which grow round the mouth in the order Thecosomata. Another portion of the foot - the mid-foot - forms two lobes which usually fuse together, and constitute the siphon: and a third portion - the hind-foot - is represented by the valve of the siphon.

The homologies of these three divisions of the foot are by no means certain, as the development of this region of the body is apparently greatly altered by the presence in it of a plentiful food-yolk.

1 An account by Herdman of Larvacea, Pyrosoma, and Thaliacea will appear in a forthcoming volume of the Challenger Reports.

The shell is essentially a cuticular structure, calcified by Lime carbonate with the exception of a thin superficial layer, the periostracum or epicuticula; but occasionally calcification fails to take place. In Di-branchiate Cephalopoda the shell is developed when present in a sac formed by the closure of two dorsal integumentary folds. In Spirula alone do these folds fail to unite. The way in which the shell is first formed in Tetrabranchiate Cephalopoda is unknown. In all other Mollusca the larva possesses a shell-gland developed as a pit of the epiblast on the dorsal aspect of the visceral dome. This gland rarely persists in the form of a sac, as in a few Pulmonate Gastropoda, but it is usually everted, and on this everted surface the shell commences its development. Its subsequent growth is dependent on the mantle. It extends with the extension of the mantle-edge, and an increase of thickness takes place from the surface of the mantle already covered. In some instances, e.g. Tetrabranchiate Cephalopoda and a few Gastropoda, the animal as it grows in size, quits a portion of the shell which is then closed off by a calcareous septum, The processes and ridges of the shell are formed by folds and processes of the mantle-edge, its colours by pigment-secreting glands in the same region.

The actual shape of the shell is very variable. It is composed of a single piece, except in the Gastropoda Isopleura where it consists of a longitudinal series of plates one behind the other, and the Lamellibranchiata where it consists of a right and left valve, connected, however, by a median dorsal ligament to be regarded as an uncalcified portion of the shell.

The dorsal or antipodial region of the body has a thin integument, and constitutes the visceral dome or hump. It is scarcely marked in Lamellibranchiata, is co-extensive with the foot in some Gastropoda, in others developed along a dorso-pedal axis, but with rare exceptions it is in this Class, whether large or small, spirally twisted from behind forwards along the right side of the animal. There is a consequent displacement of the ctenidia, viscera, and the nephridial and anal apertures. In Cephalopoda and Pteropoda it is also much enlarged but along an oblique axis, which is directed backwards between the dorso-pedal and antero-posterior axes. A spiral torsion of this dome is observed only in one family of Pteropoda, the Limacinidae; but whilst the Cephalopoda have retained bilateral symmetry it is almost certain that the Pteropoda have acquired it. A head is well-developed and distinct in Gastropoda Anisopleura and Gymnosomatous Pteropoda. It is obscured in Cephala-poda by the growth round it of the foot.

In other Mollusca it is more or less rudimentary.