133 THE BOOK OF NATURE STUDY 



slowly by movements of these, others crawl over the surface by 

 attaching and loosening the suckers with which their arms are 

 so abundantly furnished, but all possess the power of rapidly 

 jerking themselves backward on an alarm by ejecting water 

 from the so-called siphon, a tube placed not far from the head 

 and communicating with a space known as the mantle chamber. 

 As one of the big cuttles will readily show, this mantle chamber 

 is a comparatively large space containing the two feathery, 

 plume-like gills. It can be widely opened to allow for the 

 entrance of water, and is then capable of being closed by a curious 

 hook-and-eye arrangement. When this is done the only exit 

 is through the siphon, and it is by this that the water is suddenly 

 ejected, with the result that the animal shoots backward. The 

 eyes are large and prominent, and within the mouth two large 

 parrot-like jaws lie concealed. The rows of suckers on the arms, 

 which themselves number eight or ten, are worth careful note. 

 These arms are really equivalent to parts of the foot, that is, of the 

 flat surface on which, .g., the garden-snail creeps. The cuttles 

 are not only the highest of the molluscs, but also of the inverte- 

 brates, and show many peculiarities of structure. 



From the cuttles we pass to the bivalves, of which not a few 

 forms will live well in captivity. Among these we may begin 



with the edible mussel, 

 small specimens of which 

 make pretty inhabitants 

 of the aquarium. The 

 dark blue shell is prob- 

 ably familiar to all, no less 

 than the habit of grow- 



v ing in companies to form 



b mussel beds. Tear off a 



FlG. 64. The edible mussel (Mytilus edulis). f ew sma ll Specimens and 



P* th em in a d ish wi th 



water. As they grow accus- 

 tomed to their new surroundings they will be observed to open 

 their shells, which close tightly on an alarm. From the gaping 

 shell each protrudes a small whitish foot by means of which the 

 mussel can slowly creep along, and also the fringed margin of the 



