486 THE ANIMAL KINGDOM. 



inwards forming a deep bay at the posterior end of the valves. These two 

 conditions are known respectively as integro- or sinu-palliate. 



The cerebro-pleural ganglia usually lie at the sides of the mouth 

 connected by a dorsal commissure. They are sometimes fused dorsally 

 (Teredo}. The pedal ganglia are always closely united. Their size varies 

 with the development of the foot and they are absent in the Ostreidae, but an 

 infra-oesophageal cord passes from one cerebro-pleural ganglion to the other 

 and thus represents the pedal system. The pedal connectives are sometimes 

 excessively short and the ganglia approximated to the cerebro-pleural as 

 in Pecten. The visceral ganglia are always large, their connectives long, 

 and the two are closely united. They lie on the ventral surface of the 

 posterior adductor muscle, and give off branches to the viscera. The two 

 branches to the ctenidia are each connected with an osphradial ganglion, 

 and when the siphons are large, e.g. Solen, My a, there are siphonal ganglia 

 developed on the siphonal nerves. Both cerebro-pleural and visceral ganglia 

 give off pallial nerves, which traverse the margins of the mantle and may 

 unite, e.g. Ostrea, to form a circumpallial nerve, or are resolved into 

 a circumpallial plexus, e.g. Anodon. Nerve and plexus alike contain 

 ganglionic centres. The chief ganglia are often orange coloured. 



The tentacles or papillae developed on a greater or less extent of 

 the margin of the mantle, and at the siphonal apertures in many Lamelli- 

 branchiata, are probably tactile. Cells ending in sensory hairs are found 

 upon them in Pecten, Anomia, &c., as well as on the margins of the mantle 1 

 (p. 137). Eyes provided with a lens are often found in the larva. They 

 are two in number and lie at the base of the velum on each side of 

 the oesophagus near the auditory sacs. Teredo has a row of such organs 

 in front of the foot. But in the adult eyes are confined to the edges of 

 the mantle and the siphons. They have the form of diffuse ommatidia, 

 which are generally present and are sometimes the only kind to be found, 

 e.g. Ostrea, Avicula, Cardita sulcata; or else the ommatidia are aggregate, 

 and then the eye is either pseudo-lenticulate, evaginate, or invaginate. 

 The first-named is found only in Area ; the retineum is slightly invaginate 

 but the retinidial elements form a prominent convex lens. The evaginate 

 type occurs in Area and is the only kind in Pectuncidus. There is an 

 ommateum ; the retinophorae are surrounded each .by eight [retinulae, 

 four forming an outer, four an inner row, and the retinophoral rods are 

 prominent and convex at their outer extremities. The invaginate eye 

 is met with in Area as a simple pit ; in Pecten, Spondylus, and Cardium 

 the pit becomes a vesicle and closely resembles the Vertebrate eye. In 

 Area the pit is lined by a retineum, and the retinidial cuticula is thickened. 



1 For the structure of the hypodermis, the intercellular nerves, the plexus of nerve-fibrils in the 

 inner layer of the cuticle, and the sense-organs of the tentacles in Pecten, see Patten, Mitth. Zool. 

 Stat. Naples, vi. pp. 604-5, 662-5, with the figures given. 



