LAMELLIBRANCHIATA. 21 



are frequently (Unio) far removed from one another laterally, and are 

 approximated to the anteriorly placed pedal ganglia (Pecten), whose 

 nerves are distributed on the ventral side of the body in the foot. 

 The large visceral ganglia are placed on the ventral side of the 

 posterior adductor muscle, and supply nerves partly to the gills and 

 partly to the viscera and to the mantle ; the nerves supplying the 

 latter are two large trunks which run in the edge of the mantle and 

 anastomose with the mantle nerves from the brain, often forming 

 plexuses. Large nerves also pass off from the visceral ganglia to the 

 siphons, at the base of which they form an accessory pair of ganglia. 



Sense organs. Auditory organs, eyes, and tactile organs are 

 present. The former have the form of paired auditory vesicles, and 

 lie beneath the oesophagus attached to the pedal ganglia (their 

 nerve, however, arises from the brain) ; they are characterised by the 

 large hair-cells which line the wall of the vesicle. Eyes may either 

 be simple pigment spots at the end of the respiratory tube (Solen, 

 Venus), or be much more highly developed and placed on the edge 

 of the mantle of Area, Pectunculus, Tellina, and especially of Pecten 

 and Spondylus. In the latter genera they are placed on stalks 

 between the marginal tentacles, and have an emerald green or 

 brown red colour ; they consist of an eye-bulb with a corneal lens, 

 choroid, iris, and a well- developed layer of rods into which the optic 

 nerve passes. The sense of touch is provided for by the labial palps, 

 the edges of -the respiratory apertures (siphons) with their papilla? and 

 cirri, and also the often numerous tentacles at the edge of the 

 mantle (Lima, Pecten). In all probability the hair cells found in 

 the mantle are the seat of a special olfactory sense (tracking sense). 



The digestive organs begin with the mouth, which is placed 

 between the labial palps (fig. 498). The mouth leads into a short 

 oesophagus, into which the cilia of the labial palps drive small 

 nutrient particles received into the mantle cavity with the water. 

 Jaws and tongue are always absent. The oesophagus widens into a 

 spherical stomach, at the pyloric end of which a blind sac, which 

 can be closed up, is attached. A rod-like transparent structure 

 (crystalline style] is often found either in the above-mentioned blind 

 diverticulum of the stomach, or in the alimentary canal itself. It is 

 to be regarded as an excretion-product of the alimentary epithelium, 

 and is periodically renewed. The intestine always attains a con- 

 siderable length, is much coiled and is surrounded by the liver and 

 generative glands ; it projects into the foot and then ascends again 

 behind the stomach to the dorsal surface ; it then traverses the 



