OP SOUTHEEN INDIA. 5 



The nervous system consists of three principal pairs of ganglions ; the first, 

 or anterior, lies close to, behind, and above the mouth ; the second, or middle one, 

 is situated in the foot, and becomes more or less rudimentary with the decreasing 

 size of the latter; this ganglion supplies the ear with nerves and partially also the 

 generative organs ; the third, or posterior, ganglion is generally the largest, and 

 situated somewhat in front and below the posterior retractile muscle. The first 

 and last ganglion are never absent ; the former supplies the anterior part of the 

 body, including the frontal portiori^of the mantle, but especially the lips and 

 intestines ; the latter provides with nerves the gills and the greater part of the 

 posterior mantle, including the siphons. 



There are no special organs of touch which are principally situated in the fleshy 

 marginal portions of the mantle and in the extremities of the siphons and of 

 the foot ; the palps appear to be concerned in the action of this organ only with 

 reference to the food which passes towards the mouth. No special and permanent 

 eyes are present in any of the Pelecypoda, and the only organs representing them 

 are the occelli which have been observed at the edges of the mantle of some 

 Aecidm, JPECTiNiD^y and others. As regard their structure, these occelli, however, 

 quite correspond to the eyes of other MoUusca, and each is supplied with a short 

 special nerve. The ear consists of a pair of small cavities, situated in the 

 muscular tissue of the foot ; each has a large otholite, and is supplied with nerves 

 from the pedal ganglion. 



Most of the Pelecypoda are of distinct sexes, only some of the sessile or 

 permanently attached forms being hermaphrodites. The sexual organs are situated 

 in the posterior part of the body, close to the Bojanus'-glands and, like these, 

 generally consist of two glands. In the early stages the glands in the different sexes 

 are very similarly formed ; it is only when the spermatozoa on the one hand and the 

 eggs on the other are more developed that the sexual distinction can be recognized 

 with sufficient certainty. The fructification of the eggs appears to take place 

 through the water, with which the spermatozoa enter the body of the female ; 

 but direct observations on this point are as yet very few and not very satisfactory. 

 It is, however, certain that the eggs generally attain a very high state of develop- 

 ment in the body of the females, and that — evidently in order to be able to retain 

 the large number of eggs, — the females are often distinguished from the males by 

 a more ventricose form and a larger size. Sometimes the embryos remain in the 

 body of the mother till they attain a perfect form with a well developed shell. A few 

 species, as in Galeomma^ Lascea (= KelliaJ^ and others, are said to be viviparous. 



The development of the Pelecypoda in the e^^ from its earliest stage is much 

 similar to that of other higher organised aquatic MoUusca. With the exception 

 of the presence of a peculiar flagellum during the earliest stage of age, and the 

 occasional presence of little occelli or eyes at the base of the labial palps, there 

 is nothing peculiar in their metamorphosis, and the secretion of the shell begins 

 at a very early period of age. 



