TYPE MOLLUSC A. ,317 



quite small, though still showing plainly a spiral form ; but in 

 Limax it is represented only by a partially calcified plate, im- 

 bedded in the roof of the mantle-cavity by the closure over 

 it of a fold of the mantle. In Avion only a few isolated parti- 

 cles of carbonate of lime persist, while in Vaginula and Onchi- 

 dium all trace of it has disappeared. 



A marked characteristic of the Pulmonata is found in the 

 character of their respiratory organ. A ctenidium is entirely 

 wanting, the only trace of its existence being the occurrence 

 in some of the aquatic forms {Limncea, Pliysa, etc.), of an os- 

 phradium near the mantle-pore. Its place is taken by the 

 roof of the mantle-cavity, which receives a rich vascular net- 

 work and functions as a lung, the mantle-cavity containing 

 air which can be renewed through the mantle-pore. The 

 heart is situated far back in the mantle-cavity, its auricle 

 lying in front of the ventricle and receiving the blood from 

 the more anteriorly-situated lung, so that the relation of the 

 respiratory organ to the heart is the same as obtains in the 

 Opisthobranchs. In the immediate neighborhood of the 

 heart lies the single nephridium, opening into the mantle- 

 cavity or else into the terminal portion of the rectum {Helix), 

 this structure opening on the right side of the body in close 

 proximity to the mantle-pore. 



As in the Opisthobranchs, the rotation of the mantle-cavity 

 and its organs, as indicated by its position on the side of the 

 body, has not extended as far as in the Prosobranchs, and 

 consequently there is no crossing of the pleuro-visceral con- 

 nectives. The Pulmonates are orthoneurous. The ganglia 

 are present in typical number, and are massed together, as in 

 some Opisthobranchs and Prosobranchs, behind the buccal 

 mass. 



Special visual organs are invariably present with the struct- 

 ure which has already been described. In some forms they 

 are situated, as in the Prosobranchs, at the bases of the ten- 

 tacles ; while in others they are found at the tips of these 

 structures — ^the Pulmonates being divisible, according to the 

 situation of the eyes, into the BasommatopTiora, including such 

 forms as Limncea, Physa, Planorhis, and in general the aqua- 



