114 SWIMMING MOLLUSCA. 



it allows the shell to be raised up by the elasticity of the 

 body." The snails of spiral shells are bound to them by two 

 muscles, which arise from the pillar, and, having penetrated 

 the body below its spiral part, run forward under the sto- 

 mach, and spread their fibres in several slips, which interlace 

 with those of the muscles proper to the foot, the substance 

 of which they enter. It is obvious from this direction, that, 

 on their contraction, the body of the snail must be drawn 

 within the shell. When it wishes to reissue, the head and 

 foot are forced out by circular fibres, which surround the 

 body immediately above the foot.* The muscles, like the 

 muscles of superior animals, are composed of parallel fibres, 

 but of a bluish-white colour, -j- soft and jelly-like, and rather 

 loosely connected ; for the cellular substance, which binds 

 together those of red-blooded animals, is here very generally 

 wanting. They have, apparently, no tendons ; but this is, 

 according to Cuvier, owing to the colour being the same in 

 the tendinous and the fleshy parts. The fibres are, in 

 general, closely and inextricably interlaced, the insertions 

 being lost in one another, or in the skin under which they 

 lie, and from which, indeed, it seems impossible to separate 

 them by any definite line. Chemically they consist of 

 fibrine, but the medium which cements them to the shell 

 appears to be gelatinous, for it is loosened and detached by 

 maceration in spirits of wine and boiling, operations which 

 have an opposite effect on fibrine. J 



I. SWIMMERS. The Pteropods are the most entirely 

 natatory of all the mollusca. Created to occupy the high 

 seas, they are organized in evident aptitude to the place 

 assigned them, with a light shell, which affects not their 

 buoyancy, and with fins for progressive motion. They are 

 found far at sea, in large herds, swimming about in a lively 

 manner, by undulatory or flapping movements of their 

 membranous fins, which open and close like those of the 

 butterfly while basking in the sun. They shun the light- 



* Cuvier, Comp. Anatomy. 



t Bohadtch says, that some of the muscular fibres of the Aplysia are red. 

 De Anim. Mar. 12. 



" The muscles of mollusks either form a flat disk, or are distributed in 

 the skin so as to dilate and contract it, or are arranged about the mouth and 

 tentacles, which they put in motion. However varied the disposition of the 

 muscles mav be they always form very considerable masses, in proportion to 

 the size of tne animal, and have a soft and mucous appearance, such as is not 

 seen in the contractile fibres of the other departments of the Animal King- 

 dom. This peculiar aspect no doubt arises from the numerous small cavities 

 found in the muscles, and the mucous glands which are distributed through 

 them." AGASSIZ and GOULD'S Princ. Zoology, i. 52. 



