GASTROPODA. 91 



the gills into the auricle, and thence into the ven- 

 tricle, whence it is forced to all parts of the body. 

 The circulation is not, however, always through a 

 series of vessels, the arteries being frequently want- 

 ing, and also the veins to a certain extent. The 

 blood finds its way into cavities without walls, 

 excavated out of the flesh of the body. 



With all the species of the Class that are in- 

 cluded in the plan of this work, the respiration is 

 aquatic, and performed by means of gills. These 

 are composed of parallel plates, or of threads 

 arranged in rows, or in bundles, or more generally 

 in the form of a comb or of a feather. Whatever 

 the form, the surface is densely clothed with 

 vibrating cilia, by whose motions the water is 

 passed in incessant currents over the breathing 

 organ. The gills are generally situated on the 

 right side, in a special cavity, but sometimes, as 

 in Pleurobranchus, they lie in the angle formed by 

 the mantle and the foot, or, as in the Dorididce, 

 &c., they are quite exposed, on the surface of the 

 body. 



Although some of the GASTEOPODA are quite 

 naked, the majority are protected by a calcareous 

 shell, which is invariably, except in one family, 

 formed of a single piece, and hence is often called, 

 though somewhat incorrectly, univalve. The ex- 

 ception is that of the Chitonida, or Coat-of-mail 

 Shells, in which the shell consists of eight trans- 

 verse plates, imbedded in the mantle. The or- 

 dinary form is that of a cone, which is sometimes 

 low and nearly symmetrical, as in the Limpets, 

 but more ordinarily drawn out to a great length, 

 and rolled on itself, so as to form a spire. The 

 spire is almost always rolled dextrally, that is, 



