360 CRUSTACEA. 



the other great division of the Crustacea, the Malacostraca.* 

 This latter was applied by Aristotle to crabs, lobsters and other 

 members of the higher Decapoda, and found its application in 

 contrast, not with the Entomostraca, which were probably 

 unknown to Aristotle, but with a group of " shell-fish " with a 

 still harder shell the oysters, and other bivalves. 



Aristotle's {wa eVroyUu f included insects, arachnids, myria- 

 pods and apparently the land isopods, as well as annelids. 

 Writers of the 18th and early 19th centuries used the name 

 Crustacea for Aristotle's Malacostraca, and the name Entomos- 

 traca (first employed by 0. F. Miiller, 1785) was applied, though 

 without very direct antithesis, to a group of animals regarded as 

 distinct from the Crustacea, as thus understood, and approx- 

 imating more nearly to insects. Even so late as 1840, 

 Erichson distinguished between Entomostraca and Crustacea 

 and included in the former Limulus, together with A pus, Bran- 

 chipus, Daphnia, Cypris, the Cirripedes, Cyclops, Lernaea, etc. 

 Some time before this date, however, many naturalists had 

 recognised the necessity of establishing a comprehensive group 

 to which the name Crustacea was applied. Aristotle's name 

 Malacostraca was revived to designate, though in a larger 

 sense, the higher division of Crustacea, while that of Entomos- 

 traca was employed for the lower division, with the exclusion of 

 Limulus. It has thus come about that the names Entomostraca 

 and Malacostraca stand for the two divisions of the Crustacea, 

 though the members of the former group are not more con- 

 spicuously segmented than those of the latter, and the Malacos- 

 traca have as a whole firmer shells than the Entomostraca. 



There are not many positive characters which distinguish the 

 Entomostraca as a group. They are for the most part animals 

 of small size as compared with the Malacostraca. The number 

 of body segments, though fixed in Cirripedes and approximately 

 constant in Copepods, varies widely in the other orders. The 

 abdomen commonly terminates in a caudal fork. The excretory 

 glands of both second antennary and second maxillary segments 

 are developed in the course of the life-history, but it is the latter 

 which (as the " shell-gland " in some groups) becomes the excre- 

 tory organ of the adult a relation the reverse of that usually 



* 



/ua\a/cos soft. offrpaKov shell. 

 j- fwoj< a living thing ; ^JTO^OS cut in pieces, or, as we say, " segmented." 



