SENSE ORGANS. 



13 



connected with the cerebral ganglia by a longer or shorter commissure, 

 and give off nerve plexuses to the heart, gills, and generative organs. 

 This third pair of ganglia is, therefore, regarded as the equivalent 

 of the sympathetic, but unjustly, as it also gives off nerves to the 

 skin and muscles. Small ganglia (buccal ganglia}, lying above and 

 below the buccal mass and sending off nerves to the oesophagus and 

 intestine, may more justly be regarded as sympathetic. 



Tactile organs are present in the 

 more highly-developed Mollusca, as 

 two or four lobes, the above-men- 

 tioned buccal lobes, placed near the p ( 

 mouth. In addition to these tenta- 

 cles round the edge of the mantle 

 are often found in the Acephala, 

 and in the Ceplialophora two or 

 four retractile tentacles on the 

 head. The eyes have almost always 

 a complicated structure, and are 

 provided with lens, iris, choroid, 

 and retina. There are usually! 

 two of them on the head ; in rare 

 cases e.g., in some Lamellibranchs 

 they are more numerous, and are 

 placed on the edge of the mantle. 



Auditory organs are very generally 



\^^_^B^V r A\ w '"'"'"^f / if 

 present. They have the form of 



closed otocysts, provided with hairs 

 on their internal walls. They are 

 usually paired, and lie either on FlG - ^.-Nervous system of the pond 



A *^,, rt l / A ,,,7^^..\/'4N-rt* T^rtK/i^ f\ w.r-.t-1+V, 



mussel (Anodonta)(Kfter Keber). 0, mouth ; 

 A, anus; K, gills; P, foot; Se, labial 

 palps ; Off, cerebral ganglion ; Pg, pedal 

 ganglion; Vg, splanchnic ganglion; Gr, 

 generative gland ; On', external opening 

 of kidney; Oe", opening of generative 

 gland. 



the cerebral or pedal ganglia (fig. 

 497, Ot). They are, however, al- 

 ways innervated from the former. 

 In the alimentary canal, three 

 divisions, at least, can be clearly 

 distinguished the oesophagus, the stomach and intestine, and the 

 hindgut or rectum. Of these the middle or digesting division 

 (stomach and intestine) is usually characterized by the possession 

 of a very extensive liver. Kidneys are always present, and are 

 frequently paired and symmetrical in each half of the body. Often, 

 however principally when the body is asymmetrical the kidney 

 of one side is smaller (Patella, Haliotis) or is entirely absent (Gastro- 



