GASTEROPODA. 99 



The development of the pulmonifcrous embryo is best seen in the trans- 

 parent eggs of the fresh-water Kmneids ; these are not hatched until the young 

 have passed the larval condition, and their ciliated head-lobes (or veil), arc 

 superseded by the creeping disk, or foot. 



The shell of the gasteropods is usually spiral, and univalve ; more rarely 

 tubular, or conical, and in one genus it is multlvalve. The following are its 

 principal modifications : 



A. Regularly spiral, 



a. elongated or turreted ; terebra, turritella, 

 h. cylindi-ical ; megaspira, pwpa. 

 €. short ; buccimirn. 



d. globular; natica, helix. 



e. depressed ; solatium. 



f. discoidal; planorbis. 



g. convolute; aperture as long as the shell ; cypraa, bulla. 

 h. fusiform ; tapering to each end, like fusus. 



i. trochi-form ; conical, with a flat base, like trochus. 

 k. tm'binated ; conical, with a round base, like turbo. 

 I. few-whiiied; helix hmnastoma. PI. XII., fig. 1. 

 m. many- whirled ; helix polygyrata. PI. XII., fig. 2. 

 n. ear -shaped ; haliotis. 



B. Irregidarly spiral ; siliquaria, vermetus. 

 C Tubular; dentalium. 



D. Shield-shaped ; umbrella, parmophorus. 



E. Boat-shaped ; navicella. 



r. Conical or limpet-shaped ; patella. 



G. Multivalve and imbricated ; chiton. 



The only symmetrical shells are those of carinaria, atlanta, dentalium, 

 and the limpets.* 



Nearly all the spiral shells are dextrul, or right-handed ; a few are con- 

 stantly sinistral, like clausilia ; reversed varieties of many shells, both dex- 

 tral and sinistral, have been met with. 



The cavity of the shell is a single conical or spiral chamber ; no gastero- 

 pod has a multilocular shell like the nautilus, but spurious chambers are 

 formed by particular species, such as trito7i corrugatus (fig. 62), and euomphalus 

 pentangulatus ; or under special circimistances, as when the upper part of the 

 spire is destroyed. 



Some spiral shells are complete tubes, with the whirls separate, or scarcely 



* The curve of the spiral shells and their opercula, and also of the Nautilus, is a 

 logarlUimic spiral; so that to each particular species may be annexed a number, indi- 

 cating the ratio of the geometrical progression of the dimensions of its whirls. Rer. 

 H. Moseley, " On geometrical forms of turbinated and discoid shells." Phil. Trans. 

 Lond. 1S38. Pt. 2, p. 351. 



P2 



