THE PHYLUM MOLLUSCA 



249 



ture richly supplied with blood vessels. Along the upper edge of the 

 gill numerous glands secrete mucus that passes over and cleanses the 

 gill. Water leaves the mantle cavity through the slit between the ante- 

 rior edge of the mantle and the head. 



Blood passing through the capillaries of the gill is collected in a 

 large vein that empties into the single auricle. The heart, which lies close 

 to the intestine, is reversed during torsion so that the ventricle lies 

 posterior to the auricle. In Busycon canaliculatum, which twists to the 

 right during development, only the left gill and the left auricle develop. 



The single left nephridium lies over the heart, opening dorsally into 

 the mantle cavity. Most of the blood passing through the nephridium 

 goes through the gill, but a small portion goes directly to the auricle, 

 so that the final mixture is not completely oxygenated. 



Most of the nervous system is centralized anteriorly. Except for 

 one pair on the intestine, the ganglia are located close to the brain, 

 forming an irregular ring around the esophagus (Fig. 14.5). The several 

 elements can be distinguished by careful dissection. The visceral gan- 

 glia on the intestine were reversed during torsion, so that their con- 

 nectives with the rest of the system are crossed, a persistent feature 

 characteristic of many snails. 



The head bears a pair of sensitive tentacles, and halfway out on 



Mantle. ; 



Mucous ^land— ^ 



ChcTDOTXce-ptor,^ 



Siphon- 



Tentacle'' , 

 Proboscis — \- 



•Nephridium Anus 



N^ pDigestive \, 



PVoboscis- 



rRectum. /"Intestine 



Esophagus 



Brain. 



^Auricle 



Y'-'-.ricardial 

 cavity 

 I Ventricle 



Digestive 

 gland 



"Stomach 



Mouth" 



Nephridium" 

 • / 

 Dibestive"; 

 gland I 



Ovary — ^ 



Stomach 



Operculum Lj^adula. 



-Shell ^land ^x~ 



-VVagina / /' 



( \ /l)igestivA 



Nephridium 



;Posterior linrjib of 

 , >v mantle cavity 



VVSperm 

 y \ groove 



Retractor 

 muscle 



Pe-nis" 



"Stomach 



Testis 



Figure 1 4.5. Anatomy of Busycon canaliculatum (shell removed). A, Left side, show- 

 ing external organs and internal organs visible through the integument. B, Same view 

 with digestive, respiratory, circulatorv and nervous systems indicated. C, Female, show- 

 ing portion of the right side. D, Male, portion of the right side with mantle and retractor 

 muscle cut short. In C and D the proboscis is withdrawn. Adult with shell is shown in 

 Figure 14.2. 



