NATURAL HISTORY OF ARTHROPODS. 



Class I. — CKUSTACE A. 



The Crustacea as a group are essentially aquatic, and although some of the members 

 live on the land, all require the presence of moisture for the purposes of respiration. 

 Nine-tenths of the known species live in the sea, while the majority of the remaining 

 forms inhabit fresh water, only a very few bemg adapted for life on the land. In size 

 they vary from forms only to be seen with the microscope to the giant Macrocheira of 

 the Japanese Seas, whose legs will occasionally embrace a distance of twenty feet and 

 even more, and the lobster of our own coast, specimens of which have been taken 

 weighing forty pounds. No very reliable or accurate estimates have been made as to 

 the number of existing species, but probably ten thousand is within the limits. This 

 number is much larger than the one usually assigned, but when we recollect that there 

 are about eight hundred species of Decapoda alone described from North America and 

 the West Indies, it will readily be seen that om- estimate for the class is certainly within 

 bounds. 



The body of the Crustacea is almost universally enveloped in a more or less 

 hardened chitinous integument, in which, in the Barnacles and the higher groups, 

 carbonate and phosphate of lune are deposited, giving this external skeleton much 

 greater firmness. This, though a character of but slight morphological importance, has 

 nevertheless given the name to the class in allusion to the crustaceous character of the 

 body walls. Were this external skeleton solid and firm all motion would be impossible, 

 but this is provided for by joints in which no lime is deposited, and which are there- 

 fore softer and more fiexible. As in all Aithropoda, we can reduce the body to a series 

 of rings or somites, arranged one after another, and each typically bearing a pair of 

 jointed appendages. So far this corresponds with the structure of the insects, but it is 

 to be noticed that in the Crustacea each appendage consists of a basal joint {basiqp- 

 odite) attached to the body, and from this arise two jointed branches, the inner being 

 called the endopodite, the outer the exopodite, the inner and outer feet. In the adult 

 . forms of many of the Crustacea but one of these branches persists in some of the limbs 

 of the adult, though in the young the bifurcate character is almost always plainly to be 

 seen. By following through the development we find that it is the outer branch which 

 has disappeared in the adult. In the anterior portion of the body the rings are fi"e- 

 quently so completely coalesced that it would be difiicult to ascertain their number 

 were it not for the morphological law first propounded by the eminent French natural- 

 ist, J. C. Savigny, that each segment of the arthropod body bears but one pair of 

 appendages ; a law to which, however, there are several exceptions. The number of 

 segments in the Crustacea varies widely, from the three indicated segments of thelarv83 

 of some forms to twenty segments in the Decapoda, and forty-seven in Apus, one of 

 the Phyllopods, which, by the way, affords one of the exceptions to Savigny's law, it 

 having twenty-seven thoracic segments bearing sixty pairs of limbs. 



^As has been said, it is frequently difiicult in certain portions of the crustacean 

 body to make out the limits of some of the segments, and especially of those in the 

 anterior part. This is due to two causes : the segments are frequently coalesced so 

 that the sutures are almost obliterated, and partly to the fact that certain segments are 

 so hyperti"ophied that atrophy of parts of the adjacent somites of a necessity follows. 



