xm] 



HELIX 



287 



cilia which cover the gill. A gill of this nature is called a 

 ctenidium, owing to its coinb-like appearance (Gr. /crevi'Siov, a small 

 comb). Now, since the snail breathes air, not water, it has lost the 

 gill, but to compensate for the loss it has changed the whole mantle- 

 cavity into a lung. The floor of the mantle-cavity, really the back 

 of the neck, is arched and composed of muscles : when these con- 

 tract the floor flattens and thus the mantle- cavity is enlarged and 

 air is drawn in. 



The blood is contained in 

 large vessels running in the 

 thin roof of the mantle-cavity : 

 these are clearly seen when the 

 mantle flap is clipped away 

 from the neck and turned over 

 to the right (8, Fig. 128, and 

 Fig 129). These vessels are 

 seen to all converge to the 

 heart, which consists of two 

 small oval sacs placed end to 

 end. That into which the vein 

 enters is thin-walled and is 

 called the auricle: the .other 

 thicker one is called the ven- 

 tricle (Fig. 129) ; it is the more 

 muscular of the two and drives 

 the blood through two arteries 

 to the body. One of these passes 

 up to the visceral hump, and 

 the other forward to the head 

 and neck. In Molluscs which 

 have gills the auricle always 

 receives the blood from the gill : 

 when there is one gill, as is the 

 case with nearly all the uni- 

 valves, there is only one auricle : 

 but where, as in the bivalves 

 and cuttle-fish, there are two or 

 even four gills (as in Nautilus) 



there are likewise two or four auricles. The heart is surrounded 

 by a space called the pericardium, which really corresponds to the 

 body-cavity or coelom of Vertebrates, Annelids and Echinoderms, 



FIG. 128. Helix pomatia. The animal 

 seen from the dorsal side after removal 

 of the shell. From Hatschek and Cori. 



1. Auricle of the heart receiving pul- 

 monary vein. 2. Anterior tentacles. 



8. Eye tentacles. 4. Edge of mantle. 

 5. Kidney. 6. Liver. 7. Al- 

 bumen gland. 8. Pulmonary vein. 



9. Foot. 



