CLASSIFICATION OF THE ANIMAL KINGDOM. 179 



Between the lobes of the mantle are found four delicate leaf-like organs, 

 which are used for the purposes of respiration, and are called the branchiae; 

 they are covered all over with innumerable vibratile cilia, which, by the 

 currents they cause, serve the double purpose of aerating the blood, and 

 bringing food to the mouth. At the base of the branchial organs is situated 

 the mouth, a simple orifice provided with thin lips, without any dental 

 apparatus. Near the oral aperture is placed a fleshy retractile organ called 

 the foot, which in some species can be protruded to a great length. By 

 means of this instrument many Mollusks can burrow in the sand with great 

 rapidity; others use it as a sucker to keep them in the position which 

 they have chosen to live in; and others employ it as a means of loco- 

 motion, by springing with it from place to place; while Byssiferous Mollusks 

 construct with it the cables by which their shells are attached to rocks 

 or other marine substances. The remaining internal parts of the Conchifera 

 are the stomach, which varies considerably in different species, the liver, 

 which is usually of a large size, the generative apparatus, which is simple 

 in its structure, the intestine, which is long and convoluted, terminating 

 at the opposite end of the shell to where the mouth is situated, and the 

 sart, which also varies in different species, some possessing an auricle and 

 a ventricle, others two auricles and one ventricle, while others again, as 

 in Area, have four distinct cavities, two auricles and two ventricles. The 

 valves of the shell are connected posteriorly by a ligament or hinge of 

 various formation, which, by its elasticity, serves to keep them apart; and 

 the antagonist to this force is the adductor muscle, which passes interiorly 

 from one valve to the other, and is of great strength. Some species possess 

 only one of these* muscles, and are named Monomyaria; others (and they 

 are the most numerous) have two, and are termed Dimxjaria. The nervous 

 system, as would naturally be supposed from the contracted habits of this 

 class, is not very highly developed; a few small ganglia, with nervous fila- 

 ments radiating from them, have been discovered in various parts; one pair 

 is placed in the proper position of the brain, just above the oesophagus; 

 others are found in the muscles, and near the vicinity of the branchial 

 organs, those of the Dimyaria being the most highly developed, in proportion 

 to their increased muscular powers and general structure. 



The fifth class, Gasteropoda, consists of animals which live either in the 

 sea or on land, and is very numerous. Most of them inhabit univalve 

 shells, a few, as the Chitons, have a covering composed of several pieces, 

 generally eight, while many others have no protection at all. "From the 

 superiority of their mode of progression," says Professor Jones, "it is evident 

 that they are adapted to enjoy a less limited intercourse with external 

 objects than even the most highly gifted of the burrowing Conchifera," 

 and we therefore find a proportionate increase of development of the nervous 



