200 COMPARATIVE ANATOMY CHAP. 



The original right gill and right auricle are usually retained in Gastro- 

 pods with shells dextrally twisted. In Gastropods with a true sinistrally 

 twisted shell, the left gill and left auricle are retained. 



There is, however, a whole division of the Prosobranchia, the 

 Diotocardia, in which both auricles are retained. It is evident that the 

 gills are more liable to disappear than the auricles, since in some 

 groups both auricles remain when one gill has disappeared (for 

 details see opposite page). 



When, in Gastropoda with only one auricle, the pallial complex has 

 shifted to the anterior side of the visceral dome, the respiratory organs 

 lie in front of the heart, and the single auricle in front of the ventricle 

 (Prosobranchia, Monotocardia, most Pulmonata, a few Opisthobmnchid). 

 In those Gastropoda, however, in which the pallial complex lies on one 

 (usually the right) side of the body, the gill is placed behind the heart 

 and the auricle behind the ventricle. This is the case in nearly all 

 the Opisthobranchia. In a few Pulmonates also, such as Testacella, 

 Oncidium, etc., the auricle lies behind the ventricle, as a consequence 

 of special organic modifications. 



The blood, or rather the haemolymph, is a fluid rich in dissolved 

 albumen (hsemocyanine), which assists in nourishing the body and in 

 respiration. Amceboid cells, the lymph cells or amcebocytes, are 

 suspended in the hsemolymph. Haemoglobin is occasionally found 

 dissolved in the hsemolymph or combined with special blood corpuscles. 

 The lymph cells either become detached from the wall of localised 

 blood-making glands, which may vary in position, or, in a more 

 diffused manner, from large vascular areas. They seem, from their 

 origin, to be cells of connective tissue. 



The walls of the heart and of the walled vessels consist of smooth 

 muscle fibres thickly felted, and (on the heart) of an external endo- 

 thelium which belongs to the pericardium. An inner endothelium is 

 wanting, so that the muscle fibres are directly bathed by the blood. 



The wall of the ventricle is always more muscular than those of 

 the auricles. At the point where the auricles open into the ventricle, 

 valves projecting into the lumen are always found, which, when the 

 latter contracts, prevent the return of blood into the auricle. Besides 

 these atrio-ventricular valves, there are occasionally other valves 

 between the ventricle and the aorta. Valves may also occur in the 

 peripheral blood channels, when these form contractile enlargements 

 (e.g. the valve between the branchial heart and the afferent branchial 

 vessels of the Cephalopoda). 



In various Gastropods and in Chiton a network of ganglion cells 

 and nerve fibres has been found in the wall of the heart, innervated 

 by two nerves of different origin. The nerve which runs to the 

 ventricular plexus originates, in the Prosobranchia, in the left parietal 

 ganglion, that running to the auricle from the left parieto-visceral 

 connective. Where there are two auricles, they are innervated from 

 the branchial ganglia. 



