THE LOOPING SNAIL. 327 



The Common, or False Wentletrap, is tolerably common upon European coasts. 



In this shell the whorls are united together and furnished with a number of circular eleva- 

 tions, which, however, are not nearly so bold as those of the preceding species, but thick in 

 proportion to their height, set obliquely on the shell, and smooth. 



The animal has a proboscis-like mouth, which can be retracted at the will of the owner ; 

 the tentacles are tolerably long, placed near together ; and the eyes are set near the base of 

 the tentacles. The foot is triangular, with the front rather obtuse, and supplied with a fold. 

 When disturbed or alarmed, the creature is capable of exuding a dark purple fluid. Nearly 

 one hundred species of Wentletrap are known, all the largest examples being found in tropical 

 regions. They live at a considerable depth, sometimes being captured in eighty fathoms of 

 water, and little seems to be known of their habits. 



The color of the Common Wentletrap is rather varied. Sometimes it is dull white, some- 

 times it is very pale brown, and in a few specimens the shell is reddish-violet, with the ribs 

 purple. 



We now arrive at another family, termed the Litorinida 3 , or Shore Mollusks, because the 

 greater number of them frequent the coasts, and feed upon the various algse. The shell is 

 always spiral and never pearly, by which latter characteristic it may be distinguished from 

 certain shells belonging to another family, but somewhat similar in external appearance. The 

 aperture is rounded. The animal has its eyes set at the outer bases of the tentacles, and the 

 foot is remarkable for a longitudinal groove along the sole, so that in the act of walking each 

 side advances in its turn. The tongue is rather long, and is armed with a formidable series of 

 sharp teeth, that serve admirably for the purpose of scraping away the vegetable matter on 

 which the animal feeds. The operculum is horny, and rather spiral. 



The common Periwinkle (IAtorina litbrea) is the most familiar example of this family, 

 and is too well known to need any detailed description. The Periwinkle is found upon our 

 rocks in great profusion, occupying the zone between high and low water, and always being 

 found near the edge of the tide. There is, however, another species {IAtorina rudis) which 

 occupies a rather higher zone than the previous species, and which, though very plentiful, is 

 not eaten, in consequence of its young obtaining their shells before eggs are laid, and having 

 a gritty and unpleasant effect upon the teeth. Sea birds, however, are not very particular 

 about this drawback, neither is the thrush, which, in winter, when the snails are hidden away 

 in their dark recesses, finds a meal easier to be obtained on the sea-shore than in hunting for 

 its usual prey. 



One of the prettiest members of this family is the Winding Staircase-shell, or Per- 

 spective Trociils, so named on account of the peculiar formation of its whorls. 



If the shell be held with its top downwards, it looks exactly as if it had been wound 

 around a conical centre which had afterwards been withdrawn, and the projecting edges of 

 the whorls have a wonderful resemblance to the perspective view of a winding staircase 

 seen from below\ 



Perhaps the most remarkable point about this genus is the singular operculum of some of 

 the species, which differs from that of any other mollusk. Instead of being a nearly fiat plate, 

 of horny or shelly substance, it is a conical structure of shelly matter with a riband of mem- 

 branous substance wound round it, and projecting like the mechanical form so well known as 

 Archimedes' screw. The object of this singular variation is quite unknown. 



The color of the shell is rather variable, but consists of mottlings with brown, ochre, 

 and white. 



A very curious member of this family is the Looping Snail ( TriincateUa truncdtula), a 

 little species that is remarkable for the habit which has earned for it its popular name. All 

 these creatures inhabit the space between tide marks, and can live for many weeks without 

 water. Their mode of progression is very peculiar, and closely resembles that of the leeches 

 or looping geometric caterpillars with which we are so familar. When they walk they fix the 

 head firmly, then draw up the body in an arch, fix the foot, and then push the head forward. 

 The foot is short and rounded at each end. 



