GAMMARIDEA. 825 



Subtribe II. GAMMARIDEA. 



The subdivision of the Gammaridea into families depends partly on 

 the adaptation of the thoracic legs to different modes of locomotion, 

 and partly on the character of the abdomen, which is also an organ of 

 motion, and undergoes wide variations. Species with the habit of 

 certain Anisopocls, naturally stand apart from the typical families or 

 groups. 



The species that have pre-eminently the narrow elongate body and 

 habit of Tanais and Caprella, are the DuLiCHiDiE ; the abdomen is 

 abbreviated, the six posterior legs are fitted for clinging, Caprella-like, 

 while the anterior legs are used for taking its food. They are, in 

 fact, but a grade removed from certain Caprellids — the Cercopes — in 

 which the abdomen has four or five joints developed. Still they are 

 quite distinct; for the five joints existing have regular abdominal 

 appendages, and the thoracic branchiae are more fully developed. 

 These are the Caprelloid Gammaridea. 



A second group includes the boring Gammaridea — the Chelurimi. 

 The abdomen has the fourth and fifth segments united into a long 

 styliform joint, and the three pairs of caudal stylets are very unlike, 

 and of abnormal forms. 



The above two families are the aberrant groups among the Gam- 

 maridea. We pass now to the family of gressorial Gammarids, which 

 is subtyjncal. 



The species — the Corophhle — differ widely from the typical Gam- 

 maridea in being capable of walking on a flat surface like Isopoda, 

 and they have nearly the habit of animals of that tribe. Yet, in 

 structure, excepting the power of spreading the legs for gressorial use, 

 they are closely like the Gammarids. They are usually broader 

 species with a somewhat depressed body, and very narrow or obso- 

 lescent epimerals, while the typical Gammarids have compressed 

 bodies, often with large epimerals, which so confine the legs that they 

 could not be spread laterally, if the articulation at base admitted of 

 it. The articulations and members of the abdomen and thorax are 

 of the normal type. 



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