826 



CRUSTACEA. 



The typical Gammaridea pertain to two groups, differing in habit, 

 and somewhat in structure. 



In one of these groups — the Gammarid^i — the mandible is, with a 

 . rare exception, furnished with a palpus, and the palpus or outer branch 

 of the inner maxillae is largely developed, being two-jointed (very 

 rarely one-jointed). They are thus like the Corophidae. They 

 farther approach that group in the extremity of the abdomen. The 

 structure of this part is of unusual importance among the Gam- 

 maridea because of its use in locomotion. The Gammaridae are 

 either natatory, or imperfectly saltatory; and have not the pre-emi- 

 nent leaping power of the Orchestidae. We find, consequently, that 

 the caudal stylets are commonly unequally or irregularly projecting, 

 and rather lax in their motions; the last pair is usually two-branched 

 and elongated, and often projects beyond the others ; they are, there- 

 fore, not constructed in the most effective manner for leaping. 



In the other group — the Orchestidae — the mandibles are without 

 palpi ; the inner maxillae have the palpus small and one-jointed, or 

 obsolete; the habits of the species are peculiarly saltatory; the caudal 

 stylets project behind in such a way that the tips lie in a straight 

 line, the last pair being very short and simple, and the others in 

 order projecting beyond it, so that the whole combine together to 

 render the extremity of the abdomen peculiarly well fitted for their 

 saltatory habits ; the stylets are less free in their motions than those 

 of the Gammaridae, and are consequently calculated to act more 

 unitedly. The Orchestidae have the body much compressed, with 

 large epimerals. The Gammaridae are sometimes as much corn- 

 pressed ; but there is a gradation to species with a subdepressed body, 

 and very small epimerals, and thence by an easy transition to the 

 Corophidae. 



The Orchestidae probably rank higher than the Gammaridae. The 

 organs of the mouth in the latter may appear to be better furnished 

 for action, by the presence of mandibular palpi and more fully deve- 

 loped maxillary palpi. Yet such accessories, may be only a mark 

 of inferiority, as they show that there is something lacking in the 

 other parts, which render them necessary. The highest Crustacea, 

 the Brachyura, have no mandibular palpi; while among the Ma- 

 croura, a lower order of the Eubranchiates, such palpi are common. 



The Orchestidae are mainly land species of Gammaridea, living to 

 great extent in sand-beaches or similar places ; and they are exceed- 



