Basommatophora.] GASTROPODA. 589 



(Siphonaria). In other cases the inferior pallial lobe, situated beneath 

 the pulmonary orifice of the Basommatophora, may be transformed 

 into a branchia (Planorbidce). The auricle of the heart is usually 

 anterior, as is the case in the most archaic Opisthobranchs, and it is 

 only in the excessively detorted forms, such as Tcstacella and the 

 Onchididce, that the ventricle lies in front of the auricle. The kidney 

 usually has a more or less elongated duct or ureter. In the nervous 

 system, as a rule, all the ganglia are concentrated round the oesophagus, 

 and are closely opposed to one another ; but this is not the case in 

 some archaic Basommatophora, such as Auricula, Latia. There is 

 no longer a common genital orifice, but the hermaphrodite duct bi- 

 furcates to form a distinct oviduct of greater or less length, and the 

 primitive hermaphrodite aperture becomes the female orifice. As a 

 result of secondary changes, the orifices of the oviduct and verge may 

 be approximated, a condition found in the majority of the Stylom- 

 matophora. 



The Pulmonates never have a free larval form ; if a veliger is 

 developed, it is always contained in the egg-membranes. The majority 

 of the Stylommatophora do not pass through a veliger stage, and in 

 other forms the velum is almost always ill-developed. 



The Pulmonates are for the most part aerial, but some live, in 

 fresh water, and others (but they are exceptional cases) are marine. 

 They are distributed over the whole world, and include some 7,000 

 species, of which more than half are helicoid forms. Most of them 

 enter into a resting-stage during some part of the year in the summer 

 in hot climates, in the winter in cold climates. 



The Pulmonata are divided into two suborders 'Basommatophora 

 and Stylommatophora ; the former are generally aquatic, the latter 

 terrestrial. 



Suborder 1. BASOMMATOPHORA. 



Testaceous Pulmonata with an external shell. The head bears 

 a single pair of well-developed contractile but not invaginable ten- 

 tacles, at the bases of which are the eyes. The stomach, or at least 

 a part of it, is very muscular. The male organ is at some distance 

 from the female aperture, except in Amphibola and Siphonaria. All 

 have an osphradium (except the Auriculidce, which are terrestrial), 

 which is situated outside the pallial cavity in those* forms in which 

 water is not admitted into the lung (Lymno'ca, Planorbis, &c.). There 

 is a veliger stage in the development, but the velum is reduced. 



Fam. AURICULIDJE, Blainville. 



Terrestrial and usually maritime animals. 'j. Head ending in a snout ; 

 tentacles subcylindrical, contractile ; eyes sessile at the inner sides 

 of their bases. Mantle closed, with a thickened margin ; respira- 

 tory orifice posterior, on the right side. Foot ovate, obtuse behind. 

 The genital duct is monaulic, the verge being connected with the 



