GASTROPODS, 



369 



f 



Gleba cordata. a, Body 



Fin ; e, Proboscis. 



Mediterranean, and it probably also occurs in the Atlantic, The second species, of 

 which only the shell is known, was found in New Zealand. Cymbuliopsis and 

 Gleba are the two remaining 

 genera of this family. 



FRONT - GILLED GROUP, 

 Order PROSOBRANCHIATA. 



The Prosobranchs, which 



include the majority of 



marine gastropods and a 



few groups of terrestrial 



forms, are always contained 



within or protected by a 



shell, and organised for 



crawling. The branchial 



or pulmonary cavity is in 



advance of the heart, and 



the auricle of the latter in front of the ventricle. This important anatomical 



character, which is expressed or referred to in the term Prosobranchiata, can best 



be understood by observing the accompany- 

 ing figure representing the male of one of 

 the periwinkles (Littorina). Here the 

 respiratory organs are lodged in a chamber 

 formed by the mantle behind the head ; the 

 gills are variable in the different suborders, 

 In some forms, with nonspiral shells, they 

 are double and symmetrical, but in others, 

 and these are the majority, where the shell 

 assumes a spiral form, the gill is usually 

 single. Some of the Prosobranchs have 

 horny jaws, but others are without ; most 

 are furnished with a radula, but this is 

 sometimes absent. Characters derived from 

 this organ have been largely employed in 

 the classifications of the suborders. .The 

 sexes are separate ; the head is distinct, and 

 furnished with a pair of tentacles, sometimes 

 having the eyes at the ends, but mostly at or 

 near their base. An operculum, which is 

 unknown among the Opisthobranchs and 

 Pulmonates, except in the case of Amphibola, 

 is developed in the majority of Prosobranchs, 

 but the number of genera in which this 



appendage is absent is considerable. It assumes very different forms, and its size is 



very variable in proportion to the dimension of the aperture of the shell. In some 

 VOL. vi. 24 



ANIMAL OP MALE PERIWINKLE, WITH THE SHELL 

 REMOVED, 



a, Mouth ; &, Sexual organ ; c, Reflexed mantle ; 

 d, Vent ; e, Kidney ; /, Slime-gland ; g, Gill ; 

 h, Heart ; m, Shell-muscle ; p, Foot. 



