3356 



THE MOLLUSKS OR SHELLFISH 



as they are similarly twisted and attached to stories and other substances in the 

 same way. They are free and spiral in early life, and crawl about like ordinary 

 gastropods, but they afterward settle down and become stationary for the rest of 

 their existence. In these circumstances, a walking foot, being of no further use, 

 becomes modified into a mere support of the operculum. The animals are worm- 

 like, with a short proboscis, horny jaws, and radula, and the head supporting two 

 short tentacles, with the eyes at the base. The species are not numerous, and 

 occur chiefly in warm and temperate seas. The members of the extensive family 

 of the Melaniidcz are inhabitants of fresh water, and are abundant in all subtropical 

 parts of the globe. The shells, are not, as a rule, attractive, being clothed with a 

 dark or olivaceous periostracum. Some are long, slender, and acute, others quite 

 globular. Perhaps the most remarkable form is Tiphobia horei, an inhabitant of 

 Lake Tanganyika, in Central Africa. In the typical Melania the aperture of the 

 shell is entire, but in some of the other genera, such as Melanopsis and Faunus, it 

 is distinctly notched in front. The animal is provided with a horny operculum, 

 and many are viviparous. Hundreds of species have been described, but many, as 

 is the case in all fresh-water groups, are distinguished by very slight differences. 

 The Strepomatida? , or Pleuroceridce , are the North-American representatives of the 

 Melaniidcz of the Eastern Hemisphere, from which they are distinguished by the 

 absence of the marginal mantle fringes, and in being oviparous in their mode of 

 reproduction. In certain places they abound in such countless numbers as almost 

 to cover the bed of some of the streams in Tennessee and Alabama. About five 

 hundred forms have been recognized. The genus lo contains the largest and most 

 striking species of all t They are short, spindle-shaped shells, often with nodose or 

 spinose whorls, and with the aperture prolonged into a distinct anterior canal. 

 They are restricted to certain parts of Virginia and Tennessee. The little sea 

 snails known as periwinkles {Littorinidcz) are dwellers on the shore. They are all 



vegetarians, and occur in the Arctic and Ant- 

 arctic regions, as well as in temperate and 

 tropical shores, wherever they can find rocks 

 and stones to crawl upon. Some occur at low 

 water; others live at high-water mark, or 

 where they are only occasionally reached by 

 spring tides. Some ascend the mangrove trees, 

 and have been found hundreds of yards from 

 the sea. Four species of Littorina inhabit the 

 English coast, the commonest being the well- 

 known L. littorea, which is consumed in such 

 enormous quantities. The periwinkles have 

 horny jaws, and a very long radula, sometimes two or three times as long as the 

 animal itself; and they are all furnished with a horny operculum to protect them- 

 selves with, when retracted within their shells. The spawn of this species is illus- 

 trated in the figure. Nearly allied to the last is the genus Lacuna, in which the 

 animal has long, slender tentacles, while on the upper part of the foot there are two 

 long appendages, which extend much beyond its pointed extremity. The species 



SPAWN OF PERIWINKLE (magnified). 



