Gasteropoda.- 



-MOLLUSCA. 



-Gasteropoda. 



319 



of the lingual ribbon usually has its margins rolled 

 together and united, forming a tube, which is presumed 

 to open gradually. The new teeth are developed from 

 behind forwards, and are brought successively into use, 

 as in the sharks and rays among fishes." — ( Woodicard.) 

 ^See figures of Buccinum and Littorina — figs. 52 and 

 57. Wlicn first hatched, the youthful Gasteropoda are 

 always provided with a shell ; but in many it is con- 

 cealed when they reach the adult state, hy a fold of 

 the mantle, and in a number of species it is thrown 

 ofi' altogether at an early stage of their existence and 

 never reappears.* 



The species which respire atmospheric air, the 

 pulmoniferoiis mollusca, undergo no change as they 

 approach maturity. When born they resemble their 

 parents in all things but size. But those which respire 

 water, the brancbiferous mollusca, undergo a consider- 

 able metamorpliosis. They commence life in a small 

 . nautiloid looking shell, large enough to contain their 

 ' whole body, and having the aperture closed by an 

 operculum. The head is provided with a pair of 

 ciliated arms or fins of considei'able size, which enables 

 them to swim freely in the water, instead of crawling 

 at the bottom. At this early period of their existence, 

 all these water-breathers resemble each other, so that 

 " there is scarcely any dirt'erence between the young of 

 eoUs and aplysia, or biicciinim and Vermehisy — {M. 

 Edwards.) Development, however, soon commences ; 

 the mantle begins to secrete calcareous matter, and this, 

 mixed with the epitheliimi or layer of animal matter, 

 is moulded upon the body of the animal into the form 

 which each genus is destined to assume. In form and 

 general appearance (he shells of the Gasteropods vary 

 very much. They are usually, however, nearly spiral, 

 though sometimes tubular or conical, and except in 

 the family of Chitons, are composed of only one valve. 

 The terminology used by conchologists, will be best 

 understood by a reference to Plate 1, where the terms 

 . used are explained by the figures. 



The class Gasteropoda is a very extensive one, and 



. its division into orders and families has engaged tlie 



. attention of many naturalists. A variety of methods 



have been proposed, but it would be uninteresting to 



, the general reader to enter into details here. We shall 



, simply state the arrangement of Dr. Gray as adopted 



, in our great national natural history- repository, the 



. British Museum. In his classical " Introduction to 



, Conchology," Dr. Johnston thus expresses himself with 



regard to this system, " There can, I think, be only one 



opinion as to the great merits of this method. Its 



superiority as a whole to any previous one can scarcely 



be questioned ; and the now views taken as to the 



position of several of the families in their respective 



orders, as well as in regard of the genera which are 



made to enter into the composition of the families, 



* This is the case with the order Nudibranchiata {Gymno- 

 hi'anchiata) ; the larvse being provided with a shell and fins on 

 the head, both of which are lost soon after development begins 

 1 1 progress, the shell most probably disappearing at the tin e 

 the animals acfjnire a locomotive foot. 



nothing less than the most extensive and critical know- 

 ledge of the entire class could have suggested." 



The class Gasteropoda, Belly-ivalhers, or Snails, 

 is divided into two large groups or sub-classes, dependent 

 upon the structure of their respiratory organs. In the 

 first sub-class, the Ctenibraxchiata, equivalent to 

 the Prosobranchiata of M. Edwards, these organs 

 are in the form of branchi^ or gills ; and they are 

 comb-like, being formed of one or two series of lamellfe 

 or plates under or on the inner surface of the mantle 

 on the back of the neck (rarely round its edges). In 

 the species composing this group, the gill cavity ia 

 open, and both adults and larvs possess shells, the 

 latter being provided, in addition to these, with deciduous 

 ciliated fins on the head. They are either unisexual, 

 or hermaphrodite and self-impregnating. In the second 

 sub-class, the Heterobranchiata, equivalent to the 

 Ol'iSTiiOBRANCHiATA of M. Edwards, the respiratory 

 organs are variously formed ; some having branchire 

 or gills, which are exposed or only slightly covered by 

 a fold of the mantle ; others having a simple lung, or 

 a closed cavity lined with a network of respiratory 

 vessels, like the lungs of beasts and birds. The species 

 are all hermaphrodite, with reciprocal impregnation. 



The first sub-class, Ctenoeranchiata, is divided into 

 two orders. Thefirst, Pectimhrancliiata, is characterized 

 by the comb-like branchiie or gills consisting of only 

 one (rarely of two) longitudinal series of lamellae on 

 the left side of the mouth, over the back of the neck. 

 The gill cavity is open, the mantle edge being free from 

 the back of the neck. The animals are unisexual, and 

 the shells are spiral, andgenerally provided with a<listinct 

 operculum. The second order, Scvtibranchiata, have 

 the gills consisting of two series of laraellfe, forming 

 one or two series over the back of the neck or under 

 the edge of the mantle round the foot. The animals 

 are hermaphrodite, self-impregnating, and the shells are 

 conical, spiral, or symmetrical. 



The second large group or sub-class, the Hetero- 

 branchiata, is divided into three orders. The first, 

 Pleurohranchiala, are characterized by the branchiae 

 or gills forming a tuft on the side of the back, under a 

 fold of the mantle. The animals are hermaphrodite, 

 with reciprocal impregnation, and the shells are spiral. 

 The adult and young are both shell bearing, and the 

 latter are provided with deciduous cephalic fins. The 

 animal and foot are elongate, formed for walking. The 

 second order, Gymnobranchiata (or Nudibranchiata), 

 have the gills exposed or contractile into cavities on 

 the surface of the mantle. The animals are herma- 

 phrodite, with reciprocal impregnation, and the adults 

 are destitute of shells, the larvoe only possessing that 

 appendage. Tlie third order, Pneumobronchiata (or 

 Pulmonata), differs from all the others in the animals 

 respiring free air in a closed chamber lined with pul- 

 monic vessels. The animals are hermaphrodite, with 

 reciprocal impregnation, and both adults and larvae are 

 shell bearing, the latter being formed like the parent 

 and having no cephalic fins. 



