ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 37 



The first, or coxa, is the joint that is most important to the necessities of 

 the animal; it is the part that invariably contains the functional organ with 

 which the appendage is endowed, and is most capable of internal organic change. 

 In the lower types of Crustacea it differs little in external form from the 

 other less important joints of the same limb, and appears to become depre- 

 ciated as it corresponds with the increasing length of the ftagellum. In 

 Aniphipoda the length of this antenna is often very considerable in the deep- 

 sea genera; whereas in those that live on the shore or on land, as Talitrus 

 and Orchestia mostly do, not only do the flagclla diminish in general pro- 

 portion, but the entire organ, as an appendage, becomes enfeebled and weak, 

 arguing strongly that its higher endowments arc best capable of full develop- 

 ment under the former than the latter conditions. 



Moving upwards in the grade of animal life, in those Crustacea that pursue 

 a wholly aquatic existence this first pair of antennae, while decreasing in the 

 length of the flagellum, does so apparently by diminishing its tenuity, and 

 so condensing all its power within less extent. This also corresponds with 

 a similar change in the coxal joint of the peduncle. 



This ehango appears to bo carried to the highest extent in the short-tailed 

 genera, of which wo may find a convenient example in the common edible 

 Crab (Cancer pagurus) of the British seas. The coxa in this genus is very 

 much larger than the other joints of the peduncle, and on being opened is 

 found to contain an osseous chamber, attached by one extremity only to the 

 antcro-extcrnal surface of the outer walls of the joint. 



In the genus Maia a similar chamber, but different in form, exists ; and 

 this probably will be found to be true of all the Brachyura or short-bodied 

 forms of crabs. 



In the Macrura a chamber of a similar nature, but longer in form, cor- 

 responds with the depreciated appearance of the coxal joint of the antenna, 

 which is longer, narrower, and carries a longer and more slender flagellum 

 than the Brachyura. But the chamber in the Macrura is certainly of a 

 very peculiar character, for it is in some of the species, such as Homarus, 

 Astceus, and Paliimrus, more or less completely filled with particles of sand. 

 This sand is thrown off with the exuviations of the animal at each successive 

 moult, and is again replaced by the voluntary act of the crustacean itself. 



In some genera, such as Anchistia, Palcemon, and Lucifer, there exist 

 various forms of otolithes. 



In lower orders, such as the Amphipoda, the antenna is very simple, and 

 generally long and slender. The second filament, which in the higher 

 groups is commonly equal in length with the primary branch, is in this 

 order reduced to a rudimentary condition, and is frequently wanting in the 

 adult form, although almost invariably present in the young stage. We 

 generally find, however, that in those genera where this antenna is reduced 

 in length the coxa increases in dimensions, while the two succeeding joints 

 are less so in proportion. 



A marked exception to this is perceptible in Orchestia and Talitrus and 

 the terrestrial Isopods, where the appendage is short and unimportant, ap- 

 proximating towards a rudimentary condition. In the Hyperidians it has a 

 I endency to become enfeebled and diminutive. The tendency to variation 

 in these two widely separated forms is certainly the result of certain altered 

 circumstances which interfere with the characteristic development of the 

 organ. 



Talitrus and Orchestia. arc genera that live in an intermediate position ; 

 their habits are between the aquatic and land Crustacea, They do not live 



