Class 2. Gastropoda. Order 2. OfdsthobrancJdata. 305 



The Bupteropoda are among the most abundant and most characteristic pelagic 

 animals ; they are blind, and are specially frequent towards evening on the surface- 

 waters, both in warm and cold seas. The other group assigned to the Pteropoda, 

 the Gymnosomata (Pterota), have no shell ; they have a small, often almost rudi- 

 mentary, foot ; and move by means of two special fin-like muscular appendages, 

 which are situated anteriorly, close to the foot, but do not represent any portion 

 of it. Various organs of prehension, " arms " with suckers, etc., may extend 

 from the mouth of these animals, (Pig. 251 B) ; the Pterota are most predaceous, 

 and attack in pai-ticular the defenceless Eupteropoda, whose distribution con-e- 

 sponds with their own. Clione Umacina, a well-known species, occun-ing abun- 

 dantly on the coast of Greenland, and attaining a length of as much as 4 o/m., 

 feeds upon a Eupteropod with a spiral shell, Limacina helicina. 



Order 3. Pulmonata. 



In tlie Pulmonata, just as in tlie Opisthobranchiataj some forms — 

 the majority, indeed — carry sliells, and possess a well-developed visceral 

 hump, whilst others are naked, and have no such hump, the viscera 

 being sunk into the lower portion of the body. The two groups 

 differ, however, in that in the Pulmonata, even if the shell and visceral 

 sac be absent, the mantle-cavity is still present on the upper surface 

 of the animal as a pouch, covered by a shield-shaped mantle ; the 

 inner side of the mantle in the naked, as well as in the shell-bearing, 

 forms, is provided with a rich vascular network. The opening which 

 leads into the mantle-chamber is not, as in other Gastropoda, a broad 

 slit, but is merely a pore on the right side ; the shell never has a 

 syphonal notch, and an operculum is wanting. Between the shell- 

 bearing and the naked Pulmonata there is a complete series of 

 intermediate forms ; in some, the shell is not large enough to contain 

 the whole animal, unless the air be dry, for the molluscan body 

 increases in size in damp air ; there are others possessing a regularly 

 formed shell, which is, however, so small, that it only covers the 

 reduced visceral hump, whilst the rest of the animal can never be 

 withdrawn into it; or the visceral sac has completely disappeared, 

 and the small plate-like shell covers the mantle only ; again, the shell 

 is a small thin plate which lies beaeath the mantle ;* or it may be 

 only represented by isolated calcareous granules, which lie attached to 

 the mantle (the latter is the case, for instance, in the Great Black- 

 slug) ; or it is entirely absent. The Pulmonata live on land and in 

 fresh water, and feed chiefly upon vegetable substances. As already 

 mentioned, they breathe nir; some Presh-water Snails {Limnoeus) , 

 however, possess, especially in youth, the power of taking water into 

 the mantle-chamber, and obtaining the dissolved oxygen from it. 



* In one form (Parmacella), the young animal possesses a small external shell, 

 which, later, becomes covered by the mantle, so that the older animal possesses an 

 " internal " shell. In other forms, an " internal " shell occurs in youth, and is 

 dependent upon similar occurrences in embryological stages ; the sac in which the 

 shell lies, is an invagination of the skin. 



