CRUSTACEA. 



151 



These gills, however, are adapted for breathing air in a way not usually 

 recognized in text-books. When we come to take up the insect.-, we 

 shall find that their bodies are permeated by tubes for conveying air to all 

 the parts, and in the gills of these forms we see an outline of this structure, 

 for they, too, contain air-tubes somewhat similar in structure to, but differ- 

 ent in origin from, those of the insects. 



It is in the water that these fourteen-footed forms acquire their greatest 

 development. Everywhere the sea-shore teems with them, some being flat- 

 tened forms much like the sow-bugs, others being laterally compressed. 

 Of the latter group the beach-fleas, which are so abundant beneath the 

 masses of sea-wrack thrown up by the waves, are most familial-. Their 

 name is eminently appropriate, for they rival the true fleas in their leap- 

 ing capacities. Turn over a bit of this refuse on the beach, and they will 

 be seen in numbers, some awkwardly trying to escape by shuffling along 

 on the side of the body, others by leaping to the most astonishing dis- 

 tances. Farther down, in the region which is never exposed by the retreat- 

 ing tide, are other forms, some 

 interesting, some common- 

 place. Some play the role of 

 hermits, occupying tubes which 

 they either find or which they 

 construct for themselves, while 

 others live protected by other 

 animals. Thus when one ex- 

 amines our large jelly-fishes, 

 he is almost certain to find in 

 it some of these forms which eat a home for themselves in the tissue of the 

 disc, where they live and raise their young-. It seems strange that they 

 should escape unharmed in this home, for all around them hang the Inn- 

 tentacles each armed with its myriads of lasso-cells capable of killing 

 larger forms than these parasites. Others make their home in the body 

 of sea-anemones, passing in and out at will, and in all probability dining 

 upon the food which the anemone, at considerable trouble, had provided 

 for itself. 



Of the forms more nearly allied to the sow-bug, the celebrated gribble 

 deserves mention. It is a small form scarcely a sixth of an inch in length, 

 with sharp-cutting jaws well adapted for gnawing the hardest wood. They 

 slowly eat their way into piles of wharves, the bottoms of boats, and all 

 other submerged timber, their immense numbers making them extremely 

 destructive, rivalling in this respect the ship-worm already described. 

 There is this difference, however, between the two. The ship-worm 



Fig. 154. — Beach-flea (Gammurus), enlarged. 



