BIVALVULAR CRAWLERS. 135 



by means of a glutinous thread affixed to the surface;* and 

 many land slugs have been seen to spin a line from the 

 gummy secretion of their skin, and thus descend, from trees 

 and precipices, by a shorter route than that by which they 

 had ascended. -j- 



Our second class of Crawlers embraces the bivalved mol- 

 lusca, or at least the great majority of them, for the Mono- 

 myairians in general have no foot, and are consequently 

 incapable of locomotion. The foot, where it exists, varies 

 greatly in size and figure,;]; accompanied always, of course, 

 with corresponding variations in the mode or velocity of pro- 

 gression, but in general it is of an Fio . 2 -2. 

 oblong shape, often with a bend in 

 the middle, and more or less com- 

 pressed. (Fig. 22.) Attached to the 

 abdominal and middle part of the 

 body, more or less in front, it can 

 be moved in nearly every direction, 

 more especially forward, shortened or lengthened, bent or 

 made straight, by the action of its interlaced texture of 

 muscular fibres ; and it can be drawn at will within the 

 valves, or protruded beyond them, by other muscles, which 

 run towards different points of the shell where they are 

 inserted. Its length is often surprising. I have seen a 

 small individual of the Crenella discors put forth a foot at 

 least six times longer than the shell, which, nevertheless, 

 when not in action, was so neatly folded up and contracted 

 within it that no part was visible. 



Bivalve mollusca proceed at a rate even slower than that 

 of any snail, and, perhaps, seldom attempt the exercise, 

 unless driven by some urgent want. One species only 

 (Psammobia aurantia, Lamk.) is certainly known to creep like 

 the Gasteropods, although, from the structure of the foot, it 

 has been conjectured that some Arcae likewise do so.§ The 

 rest, when bent on change, leisurely protrude the motive 

 organ, extend it to the utmost, apply it with hesitation and 

 care to a solid surface, and then, by contracting it, as with 

 a painful effort, they jerk forwards the body and its testa- 

 ceous envelope. Now the foot is again extended in the 

 same cautious manner, and the shell again dragged forward 



* Montagu, Test. Brit. p. 227. 



f Megalomastoma suspensum has derived its trivial name from the habit 

 tin' animal has of suspending itself by glutinous threads. It is a native of 

 the woods of St. Vincent. See Swainson's Malacology, 186, fig. 29. 



% Proc. Zool. Soc. 1847, p. 21. 



§ Blainville'a Man. p. 151. The Nucula, a genus of the Areada>, is now 

 known to creep like aGasteropod. — Forbes and Hanley, Brit. Moll, ii.217. 



