139 



mussel, as well as of other bivalve mollusca, is ef- 

 fected in a similar manner, the animal protruding 

 the foot beyond the shell, and crawling along 

 upon it ; and it is furnished, also, with the same 

 kind of adhesive mucilage, for the purpose, not 

 only of steadying its steps during motion, but 

 also, as drawn out into threads under the name 

 of byssus, of preventing it, when at rest, from 

 being washed away from the rocks to which it 

 attaches itself by the tides. The motions of the 

 shell, in these animals, are perhaps instrumental, 

 as I have already observed, rather to respiration 

 than to loco-motion ; although, according to 

 Home, the oyster is capable of projecting itself 

 to a considerable distance, by suddenly closing 

 its shell. This is effected by one strong muscle 

 near the hinge. Of the other animals of this 

 class, some, like the cuttle, move by a kind of 

 arms, attached to their head ; others, like the 

 glaucus, by their gills or fins ; and others, lastly, 

 like the sea-mouse, by a kind of rudimentary 

 legs. The arms of the cuttle eight or ten in 

 number, according to the species are provided 

 with numerous tubercular suckers, by which the 

 animal attaches itself at pleasure ; but they are 

 capable of acting at times with so much force, as 

 to enable it to spring to some height out of the 

 water. The gills of the glaucus again act mere- 

 ly in the capacity of oars ; as do the hairs or 

 bristles which invest the whole body of the sea- 

 mouse, each of which, being set in motion by 

 proper muscles, constitutes, as it were, a rude 



