590 HISTORY OF SYSTEMS. 



The normal Gasteropods, although very numerous in 

 species, are a natural group, exhibiting in its families, how- 

 ever, considerable variety in the organism, which appears 

 even in the embryo at an early period of its development. 

 In some the larva is furnished with a turbinated shell hav- 

 ing the aperture closed by means of a little operculum ; over 

 the front of the head there is a large membranous veil 

 more or less deeply divided into two lobes and garnished 

 with a fringe of vibratile cilia, to make this veil an organ of 

 locomotion ;* and there is nothing to be observed that can 

 be compared to an umbilical vesicle. In others the larva is 

 naked ; the head is not furnished with natatory veils with 

 ciliated margins ; and there exists, upon the anterior part of 

 the dorsal region, a kind of umbilical vesicle. 



The Gasteropods that, in these first steps of their deve- 

 lopment, affect these two forms, present also considerable 

 anatomical and physiological differences when they have 

 come to maturity. Some are pulmonated and breathe the 

 unmingled air ; others breathe water and are provided with 

 gills. The first have been long separated from the branchi- 

 ferous Gasteropods ; but the close affinity which binds the 

 latter together has not been sufficiently appreciated, nor 

 indicated in our systems ; for in all of them these mollusks 

 are scattered throughout in a variable number of ordinal 

 divisions, and no systematist has hitherto perceived that 

 they constitute but one, although a large, group. Yet it 

 is certain that, in their embryotic condition, the different 

 families so much resemble each other that it would be 

 difficult to distinguish generically the larvae of the Eolides, 

 or of the Aplysias, from the larvae of the Buccina and of 

 the Vermetus. 



The normal branchiferous Gasteropods only differ from 

 each other when they have advanced towards their ultimate 

 forms ; but the peculiar character of the heart, whose first 

 existence is later here in life than in animals of higher organ- 

 ization, separates them into two natural groups, which ought, 

 according to Milne-Edwards, to take the rank of Orders. 



In one of these, named Opistobranches, the blood is 

 brought to the heart in a current directed more or less ob- 

 liquely from behind forwards, and the auricle is usually 

 placed in rear of the ventricle ; the respiration is effectuated 

 by the aid of arborescent or fasciculated branchiae, which are 

 not enclosed in an appropriated cavity, but are more or less 

 exposed uncovered on the back or upon the sides, towards 



* See Reid in Ann. and Mag. N. Hist. xvii. 382. 



