390 



GASTEROPODA. 



article CIRCULATION, Jig. 321, vol. i. p. 649,) 

 they form u circle around the anus. In Tritu- 

 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 Sct/ll<ca they 

 consist of little tufts irregularly disseminated 

 over the surface of the back and upon the fleshy 

 ala; projecting therefrom. In Giaucus 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 ffio/j'sthe branchiae assume the shape of long 

 riband-like lamellae 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 Iitfero-branchiuta 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- 

 chlnta have the branchiae 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- 

 spiiatory leaflets arranged in rows upon the 

 two sides ofasemi-crescentic membrane : their 

 structure in Aplysia is represented in fig. 191. 

 Each branchial lamella (, 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 branchiae 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 branchiae re- 

 semble in structure those of fishes, and are pec- 

 tinated or composed of parallel laminae 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- 



* For a figure of the branchial chamber of the 

 Buccinum undatum, and an account of the ciliary 

 movements which have been observed in many 

 orders of Gasteropoda to be connected with respi- 

 ration, the reader is referred to the article CILIA. 



sure of the animal ; but in those which are pro- 

 vided with pectinated branchiae, 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 Scutibranchiata 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 Cyclobraacliiata 

 the branchiae consist of a series of lamellae 

 placed external to the body, around the border 

 of the mantle, by the edge of which they are 

 overlapped. 



llespiration is effected in the Pulmonary 

 Gasteropoda, whether they be terrestrial or 

 aquatic, by an apparatus fitted for breathing the 

 air of the atmosphere ; the lung or pulmo-bran- 

 chia, as we may call this singular organ, con- 

 sists of a large cavity placed beneath the man- 

 tle, over the surface of which the vessels return- 

 ing the blood from the system spread in beau- 

 tiful ramifications, and from these the pulmo- 

 nary veins take their origin, collecting the blood 

 which has been exposed to the action of the 

 air, and conveying it to the heart. A large 

 orifice admits the air freely into this chamber, 

 the walls of which alternately contracting, draw 

 in and expel it at regular intervals by an action 

 precisely similar to that of the human dia- 

 phragm. In the Slugs (Limax) the cavity is 

 small, but the network of the vessels spreads 

 over its whole surface. In the Snail (Helix), 

 on the contrary, the organ is much larger, but 

 its floor only is covered with the respiratory 

 ramifications. In fig. 322, of the article CIR- 

 CULATION, vol. i. p. 649, a diagram is given 

 of this structure, and in fig. 190, (w, ,) the 

 details of its arrangement are more minutely 

 shewn; yet even in the beautiful drawing 

 of Cuvier, from which our plate is copied, 

 the minute divisions of this superb plexus 

 are but inadequately shewn. The order which 

 has been established by Ferussac, under the 

 name of Pulmonulia opcrculutu, is composed 

 of individuals classed by Cuvier among the 

 Pectiuibranchittta, to which in every cir- 

 cumstance, with the exception of the struc- 

 ture of the respiratory system, they are closely 

 allied ; these, however, breathe the air in a 

 cavity analogous to that which we have just 

 described, only differing in the position and 

 nature of the aperture leading to it, which here, 

 instead of being a rounded orifice in the margin 

 of the collar, opened and closed at the will of 

 the animal, is a large fissure placed above the 

 head, exactly as in the Pectinibranchiate order. 



Organs of circulation. Having thus de- 

 scribed the different arrangements of the 

 branchiae, we shall be enabled more readily 

 to investigate those modifications in the dis- 

 position of the organs subservient to the cir- 

 culation of the blood which are dependent 

 thereupon. Throughout the whole class, with 



