MOLL USCA—RESPIBA TOR Y ORGANS 



99 



Lungs. 



The total disappearance of the typical molluscan ctenidium is 

 characteristic of the Pubnonata, 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 eds;e 



of the mantle fold, which 

 forms the roof of this 

 cavity, unites with -the 

 nuchal integument be- 

 neath it, except at one 

 point on the right, where 

 the respiratopy 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 

 very rich in lime-secret- 

 ing glands. The inner 

 delicate surface of the 

 mantle, which forms the 

 roof of the cavity, is 

 overspread by a close 

 respiratory vascular net- 

 work. A circular vein 

 runs alonsi; the mantle 



Fig. 94.— Slightly oblique transverse section through the 

 body and shell of Helix taken just in front of the columella 

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

 no, cephalic aorta ; rjd, genital duct (uterus) ; rp, retractor 

 muscle of penis ; pliii, 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 ; rci, renal duct ; r, rectum ; hgl, hermaphrodite 

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

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

 st, stomach. 



collar. From 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 Avhich 

 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 Pulmonata, which results from the 



