GASTEROPODA. 99 



The developmeut of the pulmoniferous embryo is best seen in the trans- 

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

 have passed the larval condition, and their ciliated head-lobes (or veilj, 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 superseded by the creeping disk, or foot.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 multivalve. The following are its 

 principal modifications : 



A. Kegidarly spu-al, 



a. elongated or turreted ; terebra, turritella. 



b. cylindrical ; megaspira, pupa. 



c. short ; buccimcm. 



d. globular; natica, helix. 



e. depressed ; solarium. 



f. discoidal; planorbis. 



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

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



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

 k. turbinated ; conical, with a round base, like turbo. 

 I. few- whirled; helix hcemastoma. PI. XII., fig. 1. 

 »2. many-whirled; helix polygyrata. PL XII., fig. 2. 

 n. ear -shaped ; haliotis. 



B. Irregularly spiral ; siliqtiaria, vermetus. 



C. Tubular; dentalium. 



D. Shield-shaped ; timbrella, parmophorus. 



E. Boat-shaped; navicella. 



P. 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 dextral, or right-handed; a few are con- 

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

 tral and sinistral, have been met vdth. 



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

 pod has a multilocidar shell like the nautilus, but spm-ious chambers are 

 formed by particular species, such as triton corrugatus (fig. 62), and euomphal-us 

 pentanyulatus ; or under special circumstances, as when the upper part of the 

 spire is destroyed. 



Some spiral sheEs are complete tubes, wdth the whirls separate, or scaix-ely 



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

 logarithmic 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. Rev. 

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

 ^ond. 1838, Pt.2,p.Zb\. 



r2 



