SENS:-: oi;.\\s. 



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 (hi weal 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 placed near the 

 mouth, the above-mentioned buccal 

 lobes ; in addition to which, tenta- 

 cles round the edge of the mantle 

 are often found in the Awphnla, 

 and in the Cephalophora 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 oil the head ; in rare 

 cases e.g., in some Lamellibranchs 

 they are more numerous, and are 

 placed 011 the edge of the mantle. 

 Auditory organs are very generally 

 present. They have the form of 

 closed otocysts, provided with hairs 

 on their internal walls. They are 

 usually paired, and lie either on FlG 



mussel (A noao 



system of the 

 )(af ter Keber). 0, mouth ; 

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

 palps ; By, cerebral o-anirlion ; P//, pedal 

 ganglion; Vg, splanchnic ganglion; G, 

 generative gland ; Oe', external opening 

 of kidney ; Oe", opening oi' generative 

 gland. 



the cerebral or pedal ganglia (tig. 

 497, Of). 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 smxller (Patella, Haliotis) or i^ entirely absent ((l<t*fr<>- 



