MOLLUSOA 



99 



growth of the dorsal body-wall, is called the mantle-flap 

 (limbus palliaUs), or more shortly the mantle or pallium 

 (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 CHAMBEE. Posteriorly in this 

 space are placed the anus and the pair of nephridial aper- 

 tui'es (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 j5rocesses 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 ctbnidia 

 or gill-combs. Usually, as wiU 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 gangUon. This is Spengel's olfactory organ, which 

 tests the respiratory fluid, and is persistent in its position 

 and nerve -supply throughout the group MoUusca. We 

 propose to call it the ospheadium. 



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 stomodxum is' 

 large and muscular, the proctodssum short ; the bulk of 

 the alimentary canal is therefore developed from the met- 

 enteron or remnant of the arch-enteron after the ccelom 

 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, zff, zl). 



Dorsally to the alimentary tract the ccelom is spacious. 

 The space (C, E, s) is termed the pekicabdium, 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 coelomic 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 MoUuscan 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 (0, v). The vessel passing fore and aft from the 

 ventricle gives off a few trunks which open into spaces 

 of the ccelom, the so-called lacunae ; these are excavated in 

 every direction between the viscera and the various bundles 

 of fibrous apd muscular tissue, and may assume more or 

 less the character of tube-like vessels with definite walls. 

 Eight and left opening into the pericardial ccelom 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 coelora (C, t), and have their own 

 apertures in the 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 MoUuscs 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 



abv. a=a^'l^ i 



Fio. 2. — ctenidia of various Mollusca (original). A. Of Chiton ; J.t., fibrous 

 tissue; a.ft.'U., afferent blood-vessel; e.&.v., efferent blood-vessel; g.Z., later- 

 ally paired lamellae. B. Of Sepia ; letters as in A. C. Of Kssnrella ; letters 

 as in A. D. Of Nucula ; d, position of axis with blood-vessels ; a, inner ; 

 & and c, outer row of lameUse. E. Of Paludina ; i, intestine running parallel 

 with the axis of the ctenidium and ending in the anus a ; 6r, rows of elongate 

 processes con-esponding 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 coelomic space 

 unattached to the body-wall. These are respectively the 

 pedal NERVE-CORDS and the visceral nerve-coeds. The 

 latter meet and join one another posterioriy. A right and 

 left (D, g.v), and a median abdominal {g.ai)) 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 ganglia (g.c), 

 which are united to one another by nerve-fibres in front of 



