LAMELUBRANCHIATA. 335 



mantle, and the sensitive surface is concave ; the light being refracted on to it 

 by a cellular lens. Further, tlu-se eyes are constructed on the so-called verte- 

 brate type, the optic nerve entering the retina on the side turned towards the 

 light and running back to the rods, which are on the inner side of the retina. 



Alimentary canal. The mouth is at the front end of the body, 

 and ventral to the anterior adductor when that is present. It is 

 placed on the median bridge, which connects the two labial palps 

 (Fig. 263). These latter structures are extensions, so to speak, of 

 the margins of the mouth. They are usually bilobed at the end 

 remote from the mouth, and marked by a median groove running 

 from the cleft between the peripheral lobes to the mouth. Their 

 surface on each side of the median groove is marked by transversely 

 directed grooves leading into the median groove. The whole sur- 

 face is richly ciliated, and they are to be looked upon as food- 

 procuring organs which create currents of water, carrying the 

 floating particles of which the food of these animals consists to the 

 mouth. Jaws and tongue are always absent. The mouth leads by 

 a short oesophagus into the stomach, at the pyloric end of which 

 there is sometimes a blind sac. A rod-like transparent structure 

 (crystalline style) is often present in this diverticulum of the 

 stomach, or in the alimentary canal itself. The significance of 

 this structure is doubtful, but it is a secretion of the alimentary 

 epithelium, and is probably to be regarded as a reserve of nutri- 

 ment, for it is periodically renewed. The liver or hepato-pancreas 

 surrounds the stomach, and opens into it by a duct on either side ; it 

 also extends into the foot. The intestine is of considerable length, 

 is much coiled, and is surrounded by the liver and gonads ; it 

 projects into the foot, and then ascends again behind the stomach 

 to the dorsal surface, where it enters the pericardium and passes 

 through the ventricle. After leaving the ventricle it passes dorsal 

 to the posterior adductor muscle to open at the end of a short 

 papilla into the cloacal chamber. 



Vascular System. The heart, which is contained in the pericar- 

 dium and lies in the dorsal region slightly in front of the posterior 

 adductor, consists of a median ventricle and two lateral symmetri- 

 cally placed auricles. The ventricle is continued as an anterior 

 aorta dorsal to the intestine, and a posterior aorta ventral to it. 

 The ramifications of the aortae lead the blood into a complicated 

 system of lacunae in the mantle and in the interspaces between 

 the viscera. These represent the capillaries and finer venous vessels. 

 From them the blood passes into the large venous sinuses, the 



