]92 BRITISH MOLLUSKS. 



hatched in the ovary, and at the end of about two months the 

 young are ejected, three or four at a time, alive. 



The animal of Paludina, it may be observed on reference to our 

 vignette, has a largely dilated foot or disk, broad in front and at- 

 tenuately rounded behind ; the head is produced into a proboscis, 

 and the tentacles, unlike those of Bythinia, are rather stout and 

 cylindrical, with the eyes raised on short conjoined stalks. This 

 modification in the position of the eyes, is apparently designed to 

 make room for an organ at the outer base of the right tentacle, 

 used for conveying water to the branchial chamber. It is in the 

 form of a tubular lobe, and on the left side of the neck there is a 

 corresponding lobe, to which no particular use is assigned. In the 

 large Ampulla-rice of the tropics, affecting situations where the 

 water is more liable to be dried up, the animal is provided with a 

 double system of respiration, having, in addition to the branchial 

 chamber on the right side, a pulmonary chamber on the left side. 

 The dormant lobe of Paludina is then developed into an elon- 

 gated siphonic tube, for the passage to this chamber of the air. In 

 most of the marine water-breathing Cephals, the siphonic lobes, 

 which appear in such a rudimentary form in Paludina, are com- 

 bined into a conspicuous central tube adapted to the same use, and 

 the shell is either notched in front for its reception, as in Buccinum, 

 or extended into a long canal for its special protection, as in Murex. 



The Paludince adhere strongly by their long and wide-spread 

 foot to the places selected for attachment, but they are sensitive 

 to the touch, and readily fall. Our two British species were for a 

 long time thought to be varieties of one and the same. Lister in- 

 clined to the fancy that the differences between them are merely 

 differences of sex, that P. vivipara is the female of P. contecta. 

 Their true specific characters were first noticed in France by 

 Millet, and in England, a few years subsequently, by Dr. Gray. In 

 P. contecta, the shell is composed of five and a half prominently 

 tumid whorls, convoluted so loosely as to leave a deep umbiHcus in 

 the centre ; in P. vivipara, the shell is composed of a whorl less, 

 and the whorls are moderately ventricose, and more constrictedly 

 convoluted, the umbilicus being reduced to a mere compressed 

 chink. When very young, the shell is encircled with lines of ex- 

 tremely delicate ciliary bristles. Lamarck originally named this 

 genus Vivipara, but on feeling the impropriety of using a specific 

 adjective in the sense of a generic noun, he changed it to Paludina. 



The general distribution of Paludina over the globe, does not 



