GEOGRAPHICAL DISTRIBUTION. 



This order is distinguished by the following characters, viz. : 

 dioecious, branchiate, shell-bearing, gills in front of heart; 

 from the latter character the name of the group is derived. 

 The Cyclobranchina (Fig. 40) and the Aspidobranchina (Fig. 

 41), two suborders, are all marine forms, but under the sub- 

 order Ctenobranchina (Figs. 33 and 42) a division in which 

 all are so far specialized as to possess "a large cervical gill of 

 pectinate form on the left side, with small olfactory organ 

 (so-called rudimentary gill) ; a spiral shell is very generally 

 present ; the male possesses a penis on the right side ; most 

 are carnivorous, and possess a protrusible proboscis " (Claus 

 and Sedgwick) there are genera adapted to each of the dif- 

 ferent kinds of environment, from the abysmal to dry-land 

 zones. Some of the genera of this suborder are restricted in 

 distribution, and constitute subdivisions of higher than family 

 value. The Ptenoglossa are all pelagic. The Rachioglossa, 

 the Toxiglossa, and the Rhipidoglossa are all marine ; but the 

 genera included under the T&nioglossa are adapted to differing 

 zones of environment from one extreme to the other. This 

 group is still further specialized, and in each transverse row 

 of the elongated radula of the tongue-like rasping organ of 

 the mouth, there are usually seven plates, and two small jaws 

 are usually found at the mouth-entrance. 



There are two divisions of the Taenioglossa, the Siphono- 

 stomata, in which the opening of the shell is canaliculated for 

 the protrusion of a proboscis-like extension of the mouth ; in 

 the other, the Holostomata, the opening is entire. But when 

 we examine the genera grouped together by possession of 

 such likeness of structure, still we find in the former group, 

 of which most of the families contain marine species, that the 

 Ampullaridse are restricted to fresh-water habitat. In the 

 holostomatous division the Cyclostomidae, Cyclophoridae, and 

 Truncatellidae are air-breathers and live on land. The Palu- 

 dinidae, the Valvatidae, the Melaniidae are all fresh-water 

 species ; while the Littorinidae are marine forms, but have species 

 in the deepest part of the ocean, and others living between 

 tides ; and many other of the families of the latter group are 

 distributed through several zones. The forms of this divi- 

 sion, Taenioglossa, which are constructed to breathe air, and 



