445 



\l 



Leptopus longipes. 



the centre of gravity c backwards in the 

 direction of d e, but by the elongation of one 

 set of legs from J to h, and subsequent re- 

 traction towards _/', and the simultaneous con- 

 traction of the other set from f to h', and 

 subsequent extension towards^/', the centre r 

 v.'ill be propelled laterally in the direction of 

 a l>, perpendicular to d e. The lines a b, d e 

 represent both the magnitude and direction 

 resulting from the two movements of the legs, 

 in g f, g' f and /; f, h' J f respectively, but, 

 by changing the position of the legs, they may 

 also move obliquely. 



The Macrourous Decapods, as the lobsters, 

 are all organized for swimming ; and they have 

 accordingly been considered under that section. 



Gasteropoda. The motions of the Gastero- 

 poda are proverbially slow ; the situation and 

 structure of the muscular foot enable them to 

 traverse surfaces vertically, as well as horizon- 

 tally. The centre of gravity is supported within 

 the base formed by the disc of the foot, which 

 is organised to expand, contract, and curve in 

 every direction; also to produce a vacuum, 

 and to secrete an adhesive fluid for the pur- 

 pose of securing the stability of their position 

 on surfaces direc'ed at any degree of obliquity, 

 or on the ceilings of rooms, or roofs of buildings, 

 in opposition to the force of gravity. 



The Limaces, or slugs, and Helices, snails, 

 present in progression a crawling or gliding 

 motion. When, for instance, the Heliv poniatia 

 prepares for moving, the head, neck, and foot 

 are first protruded from the shell ; the foot is 

 next extended on the plane of position, with 

 the shell raised upon it; the muscular fibres 

 of the foot then produce an alternate contraction 

 and elongation of the successive segments of 

 its disc, commencing posteriorly and proceed- 

 ing forwards by a visible undulatory motion. 

 During these alternale elongations and contrac- 

 tions, the animal glides perceptibly from point 



to point, though so slowly that many hours are 

 required to traverse the distance of a few feet. 

 As these animals crawl up the vertical planes 

 of a glass window, the successive undulations 

 of the foot are plainly visible when viewed 

 from the opposite side. Mu'ller considers them 

 capable of producing a vacuum at various por- 

 tions of the disc of the foot, thus availing 

 themselves of atmospheric pressure in addition 

 to their adhesive mucilaginous secretions. The 

 patella, or limpet, and similar Gasteropods are 

 well known to produce between the foot 

 and plane of motion, a vacuum so powerful 

 that the shell may be broken rather than the 

 animal will suffer itself to be detached from 

 the surface to which it adheres. The minute 

 dimensions of each undulation of the foot 

 render the Gasteropods incapable of traversing 

 loose ashes or sawdust placed in their path, and 

 these means are consequently often employed 

 by gardeners 10 prevent slugs from destroying 

 the young and tender plants. 



Cephalopoda. The locomotive organs of the 

 C'ephalopodous JVlolluscs are adapted to serve 

 the triple purpose of legs for terrestrial progres- 

 sion, arms for prehension, and oars for swim- 

 ming ; the Loligo and Sepia are also furnished 

 with fins, which are placed on each side the dorsal 

 aspect of the trunk, and are the organs chiefly 

 employed when swimming. Their terrestrial 

 progression is performed by eight legs, leaving 

 the two long tentaculte free for prehension. 

 The fleshy legs of the Cephalopods are extremely 

 flexible, of various lengths, and capable of 

 moving in every direction, whilst the acetabnlte 

 enable them to lay hold of bodies with great 

 force. In walking, the head and trunk are 

 inverted, the direction of motion is retrograde, 

 and they move very slowly on solids (Jig. 240). 



Ophidia. The Ophidian Reptiles are desti- 

 tute of organs capable of supporting and carry- 

 ing the trunk in progression, and therefore they 



