VII 



MOLL USCARESPIRA TOR Y ORGANS 



99 



.p 



Lungs. 



The total disappearance of the typical molluscan ctenidium is 

 characteristic of the Pulmonata, and is connected with their terrestrial 

 life and aerial respiration. Instead of water, air enters and escapes 

 from the mantle cavity which lies either anteriorly or laterally on 

 the visceral dome, and thus the mantle cavity becomes a pulmonary 

 cavity. The free edge 



of the man tie fold, which j, f T ^' mm ? V 



forms the roof of this 

 cavity, unites with the 

 nuchal integument be- 

 neath it, except at one 

 point on the right, where 

 the respiratory aper- 

 ture, which can be closed 

 at will, allows of the 

 entrance and egress of 

 air. Along the line of 

 its concrescence with the 

 integument, the edge of 

 the mantle is much 

 thickened, forming the 

 mantle border, and is FIG. 94. Slightly oblique transverse section through the 



verv rich in lime-secret- bo<iy and sheu of Helix taken just in front of the coiumeiia 



* (after Howes), pgl, Pedal gland ; fs, lateral pedal blood sinus ; 



ao, cephalic aorta ; <jd, genital duct (uterus) ; rp, retractor 

 muscle of penis ; plm, pallial muscle, the pallial edge having 

 united with the nuchal integument ; si, salivary gland ; cr, crop, 

 or widening of the oesophagus ; s, shell ; ms, floor of the pulmon- 

 ary cavity = dorsal integument of the posterior nuchal region 

 which is covered by the mantle; sp, spermatheca = stalk of the 

 receptaculum seminis ; pli, pulmonary cavity ; pv, afferent pul- 

 monary vessels ; re 1; renal duct ; r, rectum ; hgl, hermaphrodite 

 gland or ovotestis ; I, digestive gland (liver) ; hd, hermaphrodite 

 duct ; rm, columellar muscle ; agl, albumen gland ; i, intestine ; 

 st, stomach. 



ing glands. The inner 

 delicate surface of the 

 mantle, which forms the 

 roof of the cavity, is 

 overspread by a close 



work. A circular vein 



runs along the mantle 



. 



collar. irom it spring 

 numerous fine anastomosing vessels which ramify on the mantle. 

 These vessels are again collected into larger trunks, which enter the 

 large pulmonary vein. This vein runs upwards and backwards, 

 along the right side of the pulmonary cavity, to the left of and almost 

 parallel with the rectum, and enters the auricle. The circular vein 

 contains venous blood, but the pulmonary vein conducts blood which 

 has become arterial through respiration in the vascular network, to 

 the heart. 



Since, in most Pulmonata, as in the Prosobranchia, the respiratory 

 organ and the pallial cavity in which it is found lie in front of the 

 heart, this order is prosopneumonic. An account of the opistho- 

 pneumonic condition of certain Pulmonala, which results from the 



