CLASS III. GASTROPODA, Cuvier. 



( Paracephalophora, de Blainville ; Anisopleura, Lankester.) 



THE Gastropods are specially characterized, firstly, by their asym- 

 metrical organization ; secondly, by their well-developed head ; and, 

 thirdly, by their shell, which is formed of one piece, and coiled in a 

 spiral, at least in the larval stage. The asymmetry of some of the 

 principal organs of the body is the chief characteristic of the Gas- 

 tropoda. The essential feature of this asymmetry is that the anus 

 generally lies to one side of the median plane ; that the ctenidium 

 (gill-combs), the osphradium (olfactory organs), the hypobranchial 

 gland (or pallial mucous gland), and the auricle of the heart are single. 

 or at least are more developed on one side of the body than the other ; 

 and that there is only one genital orifice, which lies on the same side 

 of the body as the anus. 



The Gastropoda are essentially aquatic animals, and the more 

 archaic species are marine ; the stylommatophorous pulmonates 

 Cydophoridce, &c. are terrestrial. 



The diet of Gastropoda varies according to the group under con- 

 sideration. Generally speaking, the carnivorous habit is due to 

 specialisation ; various forms live and feed on colonial invertebrates, 

 such as Hydrozoa, &c. Some Gastropoda are parasitic, generally in or 

 upon Echinoderrns. 



Some 30,000 species of Gastropoda have been enumerated, of which 

 20,000 belong to the present epoch, and are distributed in every region 

 of the globe. Some marine species are found at a depth of over 

 2,500 fathoms, and some Puhnonata live in the Himalayas at a 

 height of nearly 17,000 ft. above the level of the sea. Some fresh- 

 water Gastropoda exist at a depth of 180 fathoms below the surface 

 of certain lakes ; others live, in subterranean waters, and some 

 Pulmonata are found in caverns into which daylight does not 

 penetrate. Palaeontology shows that these animals were already 

 in existence in the Cambrian period, at the commencement of the 

 Palaeozoic epoch. 



