DESCRIPTION 481 



cases goes through a metamorphosis after birth, the characteristic 

 molluscaii larva being called the veliger larva. A few mollusks are 

 viviparous. 



Distribution and llubils. — Tlie majority of mollusks are aquatic 

 animals, most of them living in the sea. Many species live in fresh 

 water and many have become terrestrial animals, although the latter 

 usually require a moist environment. The pelecypods, which have no 

 radula, feed on minute organisms suspended in the water. The majority 

 of the mollusks with a radula feed on vegetal substances; all the cepha- 

 lopods, however, and very many gastropods are predacious animals and 

 a few of the latter are parasitic. The cephalopods are active and power- 

 ful beasts of prey and include among their number the largest and most 

 highly developed invertebrate animals. 



Hii^/ or?/.— Although mollusks have been well known from the earliest 

 times on account of their conspicuous shells, it was not until the time of 

 Cuvier that the foundation of the modern classification was laid. Guided 

 by his own extensive anatomical studies and those of Poli and other con- 

 temporaries, Cuvier, in 1795, subdivided the class into the three orders of 

 Cephalopoda, Gasteropoda, and Acephala, which is the foundation of their 

 modern arrangement. Linnaeus had made the mollusks one of his four 

 subdivisions of Vermes, including among them only the slugs, squids, oeto- 

 pods, and other soft, shell-less animals. The shell-bearing mollusks he 

 included in the class Testacea, which he subdivided into Bivalvia, Univalvia, 

 and Multivalvia. 



The first students of mollusks in America, as elsewhere, were the col- 

 lectors of shells. The first scientific descriptions of importance were pub- 

 lished by Thomas Say in 1817, who between that date and 1834, when he 

 died, described a large number of species. C. S. Rafinesque, during almost 

 the same period, also described large numbers of species, especially of 

 fresh-water and land mollusks. It was by these two men and certain of their 

 contemporaries and immediate followers, notably T. A. Conrad and Isaac 

 Lea, that the foundations of the present knowledge of American mollusks 

 were laid. Of the many students of mollusks of the middle of the century 

 Gould, Stimpson, and A. and W. G. Binney are especially to be mentioned. 

 Of late years especially noteworthy is the publication by the Academy 

 of Natural Sciences in Philadelphia of the Manual of Conehology, a 

 monographic work in many volumes Which will cover the entire field. It 

 was begun in 1878 by G. W. Tryon and is being continued by H. A. 

 Pilsbry. 



The phylum Mollusca contains over 60,000 living species and a veiy 

 large number of fossil ones, it being the largest phylum of animals except 

 the Arthropoda: they are grouped in 5 classes. 



