Dissected so as to show the position of the heart and the respiratory cavity.

THE shell, with the exception of a part of the columella or spire left in situ, has been removed. The foot is completely expanded and the left upper tentacle is partially protruded. A depression on the animal's right side anteriorly above the foot, marks the position of the generative aperture. The thickened collar is seen crossing the animal's dorsum. In it, on the right, is the pulmonary aperture, and below the aperture, again to the right, the columellar lobule. The pulmonary chamber has been opened by a transverse incision, and its cavity exposed by turning forward the anterior flap. Its roof is for the most part thin, and is formed by the branchial fold of the mantle, the anterior edge of which has fused, as in most pulmonate Molluscs, with the skin of the dorsum leaving only the small pulmonary aperture. Its floor is thickened and muscular, and represents that portion of the general integument which is roofed over by the fold of the mantle. The heart, composed of a thin-walled auricle and thick-walled ventricle, is seen as in dextral Molluscs, on the left side, and below it lies in this preparation the triangular pale-coloured kidney. The vessels have been injected with a red-coloured fluid from the auricle.

The right half of the roof of the pulmonary chamber is covered by vascular ramifications: the anterior part of the left side is smooth; its posterior portion corresponds to the pericardium and kidney. The pulmonary vein passes along the edge of the cut on the right side towards the auricle. It is joined, but this fact cannot be seen here, before it enters the auricle by the efferent kidney veins. The right border of the pulmonary chamber which is applied to the line of the suture in the shell is relatively thick. It is muscular, and muscular fibres pass off from it into the roof of the chamber. One of the vessels bringing venous blood to the chamber runs along this border.

When the animal is retracted into its shell by the action of the columella muscles, seen in the next preparation, air is forced out of the pulmonary chamber; conversely, when the animal expands again by the contraction of the muscular integument including the floor of the pulmonary chamber, air is drawn in But, except during the acts of expansion and retraction, interchange of air in the pulmonary chamber with the air without must take place only by diffusion.

The foot is relatively large in most Pulmonata, and simple in contour. It is divided by a transverse furrow into a fore- and hind-part only in Pedipes and Auricula brunnea. The sole of the foot is ciliated, and in Arion its lateral walls as well. A supra-pedal gland appears to be present in all Pulmonata, as well as in some other Gastropoda (Azygobranchia, Opisthobranchia). It opens above the anterior margin of the foot beneath the head. It consists of a ciliated duct extending backwards in the substance of the foot near its coelomic surface. Into this duct open unicellular glands. Sense cells have been stated to be present in the epithelium of the duct; but it is very doubtful if the gland has the olfactory function which has been ascribed to it. A caudal aggregation of mucous glands, situated posteriorly on the dorsal aspect of the foot and opening into a depression of the integument, is found in Arion ater and some other Slugs. But no Pulmonate possesses a pedal gland opening near the centre of the creeping sole of the foot, such as is found in many Azygobranchiate Gastropoda. A supra-pedal and a pedal gland coexist in the Azygobranchiate Cyclostoma elegans, which leads a terrestrial life.

The aperture of the pedal gland has often been taken for an aquiferous pore.

Many Mollusca possess the power of suspending themselves by mucous threads, and not only those of terrestrial habit but aquatic as well, e.g. Limneidae, and the marine Litiopa and Rissoa parva. In the Limacidae the thread appears to be derived from the mucus coating the surface of the body.

The position of the anal, renal, and generative apertures in front and on the right side of the body is due to the twist of the dorsal aspect of the body or visceral dome, which is characteristic of Gastropoda Anisopleura. The presence of but a single kidney and a single generative duct is nearly equally characteristic.

The pulmonary chamber must be regarded as formed by the fold of the mantle which in branchiate Gastropoda constitutes the roof of the branchial cavity lodging the gills or ctenidia. The latter are aborted, and the mantle-fold itself has become vascular and respiratory. The fore-edge of the mantle-fold in the aquatic Pulmonate Limnaeus is free, and has not undergone fusion or 'concrescence' with the dorsum, as it has in Helix and its allies. When Limnaeus inhabits the deep waters of lakes it is said to admit water to the pulmonary chamber, instead of coming to the surface at intervals for a fresh supply of air, as it does when it inhabits shallow streams and ponds. An adaptation of the branchial fold of the mantle to aerial respiration occurs also in certain streptoneurous Gastropoda Anisopleura: namely, in the Pneumonochlamyda and in the genus Ampullaria, among Azygobranchia. In the last named, which is amphibious, and is found in tropical America, Africa, and the East Indies, the left side of the branchial cavity contains the ctenidium, and is separated by a fold from the right side, the walls of which are vascularised.

The Pneumonochlamyda, to which our English Cyclostoma elegans belongs, have lost the ctenidium, and respiration is carried on solely by the walls of the branchial cavity, as in the Pulmonata, from which order the Azygobranchia differ in such essential features as the twisted character of the visceral nerve-loop, and the separateness of the sexes. The view, which is advocated by von Ihering, that the pulmonary chamber in Pulmonata is derived from the renal organ, does not appear to be tenable.

The size and extent of the pulmonary chamber vary greatly in the Pulmonata.

The pericardial cavity is a portion of the coelome closed off completely from the remainder, but communicating, as in all Mollusca where it is present, with the exterior through the nephridium, with which it is connected by a ciliated nephridial tube. The aorta divides into an intestinal and a cephalic branch, the latter passing through the infra-oesophageal collar. The system of capillaries appears to be very complete in Helixpomatia, judging from the result of injections. The vascular system of Zonites algirus has been accurately investigated by Nalepa. The arteries and capillaries have proper walls, and are lined by an endothelium. The capillaries communicate by short branches with a narrow meshwork of wide blood-spaces - the 'transition' vessels, which open into the venous spaces or the coelome by infundibular orifices, seen also in Arion ater (=rufus) by Jourdain. The venous system is represented in part by the coelome, in part by vessel-like spaces the walls of which are formed of homogeneous connective tissue with scattered nuclei, but which are not lined by an endothelium.

The pulmonary vein pulsates rhythmically in Zonites. There is in the same Pulmonate a nervous network in the walls of the auricle, of the ventricle, and aorta, in the last two instances derived from the genital nerve. Nerves are found also in the walls of the larger vessels. Ganglion cells connected with these nervous networks appear to be rare. They are found in the walls of the heart of certain marine Rhipidoglossa (Fissurella, Haliotis, Trochus), according to Haller. See Haller, M. J. ix. 1883, p. 61; cf. Dogiel, A. M. A. xiv. 1877.

The blood of Helix contains haemocyanin, a respiratory substance in which copper is present in combination with a proteid. It is colourless when deoxidised, bright blue when oxidised. Hence the coelomic fluid of a Helix exposed to air-assumes a violet tinge. The blood-plasma of the pulmonate Planorbis contains haemoglobin. Haematin is found in the liver-secretion of Helix po?natia, H. aspersa, Arion ater, and Limax. Amoeboid corpuscles occur, but rather sparingly, in the blood.

The shape of the nephridium varies in Pulmonata. It has a long tubular duct in Helix, etc., opening near the anus; whilst in other Pulmonata, such as Arion, it has a simple round opening. Its cavity is large, and its walls are lamellate. It really consists of a urinary chamber into which open acini of very large calibre. These acini are bound together externally by connective tissue. The renal cells are ciliated, and urates of Ammonium and Calcium are found in them and in the cavity of the sac. Free uric acid and Guanin occur in Zonites as well as Ammonium urate. The nephridium of Helix pomatia is supplied with arterial blood from the pulmonary chamber and by the renal arteries; by the latter alone in Zonites.

Gastropoda.Ray Lankester, 'Mollusca,' Encyclopaedia Britannica (ed. ix.), xvi. 1883. Keferstein, Bronn's Klass. und Ordnungen des Thierreichs, iii. 2. 1862-66. (Pulmonata, p. 1160.)

Pulmonata.Semper, Z. W. Z. viii. 1856-57. Leidy, in Binney's Terrestrial Air-breathing Mollusca of the United States, i. 1851, p. 198. For the Slugs, see literature to Plate v(post).

Helix pomatia.Cuvier, Memoires pour servir a l'histoire, etc. des Mollusques, 1871. (Annales du Muse'um, vii. 1806.) H aspersa. Howes, Atlas of practical Elementary Biology, 1885, Pls. xiii. xiv. Taylor, Journal of Conchology, iv. 1883, pp. 102-105. Zonites algirus. Nalepa,- SB. Akad. Wien. lxxxvii. Abth. i. 1883. Ancylus. Sharp, Proc. Acad. Nat. Sc. Philadelphia, 1883. Onchidium (Marine Slug). Bergh, Challenger Reports, x. 1884, p. 126, and M. J. x. 1885.

Muscles and locomotion of snails.Simroth, Z. W. Z. xxx. Suppl. 1878; xxxii. 1879) xxxvi. 1882.

Glands of foot.Houssay, A. Z. Expt. (2) ii. 1884. Carriere, A. M. A. xxi. 1882. Supra-pedal gland. Sochaczewer, Z. W. Z. xxxv. 1881. Id. and Simroth, ibid. xxxvi. 1882. Sarasin, Arb. Zool. Zoot. Inst. Wurzburg, vi. 1883.

Mucous threads spun by Mollusca. Martens, Z. A. i. 1878. Eimer, ibid. Tye, Quarterly Journal of Conchology, 1878.

Pulmonary chamber=a uropneustic apparatus.Von Ihering, Z. W. Z. xli. 1884; criticisms of view, see Semper, Arb. Zool. Zoot. Inst. Wurzburg, iii. 1876-77; cf. Simroth, Z. W. Z. xxvi. 1876, p. 337 et seq.

On Ampullaria.Jourdain, C. R. 88, 1879. Sabatier, ibid, and A. N. H. (5), iv. 1879.

Vascular system and pulmonary vessels.Nalepa, op. cit. supra. Apertures of capillaries in Arion ater (= rufus). Jourdain, C. R. 88, 1879. Haemocyanin, properties and distribution. Halliburton, Blood of Decapoda (Crustacea), Journal of Physiology, vi. 1885. MacMunn, Q. J. M. xxv. 1885. Haemoglobin in Mollusca. Ray Lankester, P. R. S. xxi. 1872; cf. Sorby, Q. J. M. xvi. 1876. For colouring matters in general, see Krukenberg, Vergleich. Physiol. Vortrage, iii. 1884. MacMunn, Proc. Birmingham Philosoph. Soc. iii. 1881-83. Cf. Moseley, Q. J. M. xvii. 1877.

Connection between nephridium and pericardium in Helix. O. Nusslin, Beitrage zur Anat. und Physiol. der Pulmonaten, Tubingen, 1879. Cf. Haller, Marine Rhipidoglossa, M. J. xi. 1886.