76 



NATURAL HISTORY OF ARTHROPODS. 



FIG. 103. Unciola irrorata, 

 enlarged. 



have peculiar glands in their legs, which secrete a substance which binds together the 



sand and mud in which they dwell into a tube. The mem- 

 bers of the genus Amphithoe construct such tubes, which 

 they attach to the under surface of stones, while the species 

 of Cerapus carry their dwellings about with them much after 

 the fashion of caddis-fly larvae. Unciola does not build a 

 tube, but takes any that it may find vacant. All of these 

 forms have the power of leaving their tube ; but, for the 

 most part, they are content to stay at home, their heads 

 projecting from the opening, and all of their sense organs 

 on the alert for any passing morsel of food. 



The GAMMARID^E is the largest in number of species 

 of any of the Amphipoda, Like the last its members are 

 aquatic, and when removed from the water they endeavor 

 to travel lying upon their sides, but the attempt is not very 

 successful. When placed beneath the water they are at 

 once in their element, and swim with ease, or buoyed up 

 by that medium they walk in an upright position. Their 

 swimming is mostly done upon the back or side, and is a 

 rapid but somewhat corkscrew-like motion. They have 

 the antennula? large, and frequently bearing a secondary 

 branch. The species are mostly marine, only a few being 

 found in fresh water. 

 The most prominent genus is Gammarus, numbers of which abound among the 

 Fucus or rock-weed on our shores, and in most of our inland lakes and rivers as 

 well. They do not venture out into the open water, but remain mostly among the 

 vegetation alonof the shores. Others dwell in or on the mud at the bottom. Of their 



O O 



habits scarcely anything of popular interest can be said; but though uninteresting they 

 are far from unimportant, for their vast numbers render them very prominent in 

 the food supply of fishes. 



The members of the family 

 ORCHESTID^E are the most ter- 

 restrial of the Amphipoda, and, 

 although never found but a short 

 distance from the water, there are 

 several species which will die if 

 kept too long in that element. 

 These forms are familiar to those 

 who have spent much time at the 

 seashore, as the "beach-fleas" so 

 common on the sandy beaches. 

 The ordinary observer, however, has but a very slight idea of their real abundance. 

 If he will turn over some of the sea-wrack (the dried seaweed, etc.) which forms long 

 windrows along the sand above high-water mark, he will have some conception of the 

 incalculable number of individuals. When the weather becomes so warm that the 

 windrows no longer retain their moisture, the Orchestidae burrow into the sand. From 

 their habitat these forms are of slight importance as a fish food, but they play their 

 part in the economy of nature by devouring all decaying animal and vegetable matter 



FIG. 104. Gammarus ornatus, enlarged. 



