The bile is in all cases produced from arte rial blood, and the liver is provided with but one system of veins' answering to the hepatic.
Organs of respiration.—The respiratory or gans of the Gasteropoda are found to be con structed upon very various principles, adapted to the medium which they inhabit, or the pecu liar exigencies of the individuals composing each order. Nevertheless in different groups allied by the generalities of their organization, the respiratory system is, in most instances, found to be constructed upon the same plan, and this circumstance more than any other has rendered the position and nature of the respira tory organs the most eligible basis of classifica tion. On looking over the table which we have given at the commencement of this ar ticle, the reader will perceive at once that the names by NS ich the different orders are desig nated indicate the general disposition of the pulmonary or branchial appendages, and we shall therefore follow the arrangement there adopted in considering more minutely the pe culiarities belonging to each.
The first or .iVudibranchlate order is distin guished by having the breathing apparatus per fectly exposed to the influence of the surround ing medium, which in all the genera belonging to this division is the water of the ocean; the branehie constantly assume the shape of arbo reseent tufts, placed in different situations upon the dorsal aspect of the animal. In Doris (see article CIRCULATION, fig. 321, vol. i. p. 649,) they form a circle around the anus. In Trito nia they are disposed in two rows along the sides of the animal, extending from one extre mity of the body to the other. In Sullen they consist of little tufts irregularly disseminated over the surface of the back and upon the fleshy alw projecting therefrom. In G laucus they form on each side three large and palmated fins, being used as agents of progression as well as instruments for the purification of the blood. In (Polls the branchise assume the shape of long riband-like lamellar disposed in imbricated rows ; but whatever their form their structure is essentially the same, each tuft or lamella containing the ramifications of the branchial vessels, and effecting the oxygenisation of the blood by the extent of surface which they ex pose to the action of the surrounding water.
In the IVero-branchiata the respiratory tufts or plates are arranged around the circumference of the body, lodged in a deep groove between the margin of the foot and the edge of the man tle which covers the back. The Tectibran chiata have the brandlize covered by a little fold or operculum formed by a duplicature of the skin, and generally containing a horny or calcareous plate ; beneath this are seen the re sphatory leaflets arranged in rows upon the two sides of a semi-crescentic membrane : their structure in Aplysia is represented in fig. 191.
Each branchial lamella (a, a) divides dichoto mously into smaller plates until the divisions become extremely minute; the ramifications of the arteries and veins within them being dis tributed to each are spread over an extent of surface adequate to the efficient aeration of the circulating fluid which they contain. The principal trunk of the branchial artery (c) runs along the concave margin of the crescentic membrane, while the large-venous trunk occu pies the opposite or convex border; the veins from the branchiw all terminate in this great vein, their orifices being disposed in circles, as seen at d.
The Pectinibranchiate order includes that large family of aquatic Gasteropods which are enclosed in shells, and the arrangement of the whole of their breathing apparatus is adapted to the respiration of water. The branchiw re semble in structure those of fishes, and are pec tinated or composed of parallel lamince disposed like the beards of a feather, and attached in two or three rows to the roof of a large cavity placed under the integuments of the back ; or else in some rare cases, as in the Valvata cristata, the branchia is single, resembling a pen, and floats externally.* A very material difference is ob servable between the truly aquatic species and the pulmonary Gasteropods which inhabit the water, but breathe air ; in the latter, which are compelled to come to the surface to respire, the aperture leading into the pulmonary cavity is small and furnished with a powerful sphincter, so that the air taken in is retained at the plea sure of the animal ; hut in those which are pro vided with pectinated branchim, the entrance to the branchial chamber is a wide fissure, always allowing free ingress and egress to the circum ambient fluid. Many genera of this order are provided with a special apparatus called the sy phon, for conveying the water freely into the re spiratory chamber ; this is a semi-canal formed by a fold of the right side of the mantle, and lodged in a groove projecting from the mouth of the shell; through this channel the water at all times has free admission to the gills. The respiratory organs of the Scutibranehiata re semble those of the last order, and are contained in a similar cavity, to which the water is con stantly admitted ; but in the Cyclobranchiata the branchize consist of a series of lamellar placed external to the body, around the border of the mantle, by the edge of which they are overlapped.