128 GASTEROPOD MOLLUSCA. 



lows of the sea," tlie Gasteropocl moves forward in a con- 

 tinuous manner, marking its track, in the land species, with a 

 silver line of concrete slime exuded to smoothe the asperities 

 of the road.* You cannot fail to have noticed the snail in 

 its pilgrimage ; and the aquatic tribes progress in precisely 

 the same way, whether they slowly traverse the floor of ocean, 

 or climb the rugged steeps of the rock, or stray amongst 

 their groves of sea-weed and coral. Those which, like the 

 Helices and Trochi, have conical shells flattened at the base, 

 carry them upright ; but when the shell is fusiform or tur- 

 reted, it is usually trailed in nearly a horizontal position, 

 with the point always directed backwards. The Cypraeae, 

 when they walk, cover their shell with the lateral lobes of 

 their cloak, which are very often beautifully and vividly 

 marked with various colours ; and many other mollusca 

 cover their shells more or less completely with similar ex- 

 pansions. But the Pleurotoma virgo is the most singular 

 of all in this respect. According to Argenville, when this 

 mollusk creeps it elevates and sustains its shell and cloak 

 upon a rather long peduncle or stalk, which rises vertically 

 from the back. In consequence of this remarkable position 

 of the shell, the animal tumbles over at every impediment ; 

 but it heeds not, quietly resumes its proper attitude, and 

 pursues the road, -j- And with like awkwardness the Strom- 

 bidae and some other genera, as Rostellaria and Phorus, 

 march along. The foot is narrow, but powerful and elastic ; 

 and in progression the animal places it under the shell in 

 a bent position, when suddenly, by a muscular effort, it 

 straightens that organ and rolls and leaps over and over. J 



You are aware, undoubtedly, that a snail touched when 

 on a journey instantly contracts its members, shrinks within 

 itself, and stops for a time ; and this tearfulness is a very 

 general attribute of the class, in some carried to such an 

 excess that your patience in watching is exhausted ere they 

 will again shew themselves out of their shells : 



■©« 



Or, as the snail, whose tender horns being hit, 



Shrinks backward in his shelly cave with pain, 

 And there, all smother'd up, in shade doth sit, 

 Long after fearing to creep forth again ; 

 So, at his bloody view, her eyes are fled 

 Into the deep dark cabins of her head." 



* Lister has given a very clear explanation of the action of the foot in 

 his Exercit. Anat. de Cochleis, 153. See also some remarks by Mr. Main 

 in the Zool. Journ.iii. 599. 



+ Proc. Zool. Soc. 1847, pp. 22, 23. 



% Lam. Anim. s. Vert. vii. 90. 



