636 MOLLUSCA [SCHEMATIC MOLLUSC. growth of the dorsal body-wall, is called the MANTLE-FLAP (limbus pallialis), or more shortly the MANTLE or PALLITJM (c). The space between the overhanging mantle-flap and the sides and neck of the animal which it overhangs is called the SUB-PALLIAL SPACE or CHAMBER. Posteriorly in this space are placed the anus and the pair of nephridial aper tures (see fig. 1, E). The development of the mantle-skirt and its sub-pallial space appears to have a causal relation, in the way of pro tection, to a pair of processes of the body-wall which spring, one on the right and one on the left, from the sides of the body, nearer the anus than the mouth, and are concealed by the mantle-flap to some extent (A, B, i). These processes have an axis in which are two blood-vessels, and are beset with two rows of flattened filaments, like the teeth of a comb in double series. These are the CTENIDIA or gill-combs. Usually, as will be seen in the sequel, they play the part of gills, but since in many Molluscs (Lamelli- branchs) their function is not mainly respiratory, and since also other completely-formed gills are developed as special organs in some Molluscs to the exclusion of these pro cesses, it is well not to speak of them simply as " gills " or " branchiae," but to give them a non-physiological name such as that here proposed. Near the base of the stem of each ctenidium is a patch of the epithelium of the body- wall, peculiarly modified and supplied with a special nerve and ganglion. This is Spengel s olfactory organ, which tests the respiratory fluid, and is persistent in its position and nerve -supply throughout the group Mollusca. We propose to call it the OSPHRADIUM. Passing now to the internal organs, our schematic Mollusc is found to possess an ALIMENTARY CANAL, which passes from mouth to anus in the middle line, leaving between it and the muscular body-wall a more or less spongy, in parts a spacious, CCELOM. The stomodseum is large and muscular, the proctodxwn short ; the bulk of the alimentary canal is therefore developed from the met- enteron or remnant of the arch-enteron after the coelom has been pinched off from it. A paired outgrowth of the met-enteron forms the glandular diverticulum known as the digestive gland or (commonly) liver (E, zg, zl}. Dorsally to the alimentary tract the coelom is spacious. The space (C, E, s) is termed the PERICARDIUM, since it is traversed by a vessel running fore and aft in the median line, which has contractile muscular walls and serves as a heart to propel the ccelomic blood-fluid. This pericardial space, although apparently derived from the original ccelom, is not in communication with the other spaces and blood vessels derived from the ccelom ; it never (or perhaps in a very few instances) contains in the adult the Molluscan blood or hsemolymph, and is always in free communication with the exterior through the tubes called nephridia (renal organs). The HEART receives symmetrically on each side, right and left, a dilated vessel bringing aerated blood from the ctenidia. These dilated vessels are termed the auricles of the heart, whilst the median portion itself, at the point where these vessels join it, is termed the ventricle of the heart (C, v). The vessel passing fore and aft from the ventricle gives off a few trunks which open into spaces of the coelom, the so-called lacunas ; these are excavated in every direction between the viscera and the various bundles of fibrous and muscular tissue, and may assume more or less the character of tube-like vessels with definite walls. Right and left opening into the pericardial coelom is a coiled tube, the farther extremity of which opens to the exterior by the side of the anus. These two tubes (C, u) are the symmetrically disposed NEPHRIDIA (renal organs). The GONADS (ovaries or spermaries) are placed in the mid-dorsal region of the ccjelom (C, t), and have their own apertures in tlie immediate neighbourhood of those of the nephridia. The apertures are paired right and left, and so are the ducts into which they lead ; but at present we have no ground for determining whether the gonad itself was primarily in Molluscs a paired organ or a median organ, nor have we any well-founded conception as to the nature of the ducts when present, and their original relationship br Fio. 2. Ctenidia of various Mollusca (original). A. Of Chiton ; /.?., fibrous tissue; a.b.v., afferent blood-vessel; e.b.v., efferent blood-vessel; g.L, later ally paired lamellae. B. Of Sepia ; letters as in A. C. Of Fissurella ; letters as in A. D. Of Nncula ; rf, position of axis with blood-vessels ; a, inner ; b and c, outer row of lamellae. E. Of Paludina ; i, intestine running parallel with the axis of the ctenidium and ending in the anus a ; br, rows of elongate processes corresponding to the two series of lamellae of the upper figures. to the gonads. The genital ducts of some organisms are modified nephridia, but the nature of those of Mollusca, of Arthropoda, of Echinoderma, of Nematoidea, and of some Vertebrata has yet to be elucidated. The disposition of the nerve-centres is highly character istic. There are four long cords composed of both nerve- fibres and nerve-cells which are disposed in pairs, two right and left of the pedal area or foot, two more dorsally and tending to a deeper position than that occupied by the pedal cords, so as to lie freely within the ccelomic space unattached to the body-wall. These are respectively the PEDAL NERVE-CORDS and the VISCERAL NERVE-CORDS. The latter meet and join one another posteriorly. A right and left (D, g.v), and a median abdominal (g.ab) ganglion are placed on these cords, and from them are given off the osphradial nerves which have special ganglia (g.olf}. In the region of the prostomium the pedal nerve-cords are enlarged behind the mouth, forming the pedal ganglia (g.pe), and are united by nerve-fibres to one another. From this spot they are continued forward into the prostomium, where they enlarge to form the right and left cerebral gangl-ia (g.c),
which are united to one another by nerve-fibres in front of