326 CnUSTACEA 



are almost exclusively aquatic and the arachnids and tracheates almost 

 exclusively terrestrial and aeiial animals. 



7//sf 0/7/.— Linnaeus t;ave the name Insccta to all the animals which 

 are now included under the Arihropoda, the crustaceans, spiders, and 

 myriapods being- Insecta aplera. Cuvier in 1800 created the Crustacea 

 as an independent class, and Lamarck in 1801 performed the same service 

 for the Arachnida, restricting the term Insecta to the Hexapoda and the 

 Myriapoda. The last-named group was created in 1796 by Latreille. All 

 these animals were joined by Cuvier with the Annelida to form the Ar- 

 ticulata, but in 1845 von Siebold separated the Annelida from the others, 

 making a class of them under the Vermes, and formed an independent 

 group of the Crustacea, Arachnida, and Insecta to which he gave the name 

 Arthropoda. 



The phylum contains 3 classes. 



Key to the classes of Arthropoda: 



a, Aquatic arthropods (with a few exceptions) having gills and 2 pairs of 



antennae 1. Crustacea 



tto Air-breathing arthropods (with a few exceptions). 



&i Antennae absent 2. Abachnoidea 



&2 One pair of antennae present 3. Tracheata 



Class 1. CRUSTACEA.* 



Aquatic arthropods which breathe by means of gills and have 2 pairs 

 of antennae and biramose appendages (Fig. 542). 



External Structure.— An elongated body with distinct segmentation, 

 in which the primary division into head, thorax, and abdomen is evident, 

 characterizes most crustaceans. A tendency is however present in all the 

 groups towards a fusion of the somites and a shortening of the body. 

 Those forming the head are always thus fused and are besides often 

 joined with some or all of the thoracic somites, forming thus a body 

 division called the cephalothorax, which in many crustaceans is wholly or 

 partly covered l)y a bivalve shield called the carapace. 



The appendages are fitted primarily for locomotion and respiration in 

 the water and are typically biramose, each consisting of a basal piece, the 

 protopodite, and two segmented, terminal pieces, an outer one, the exopo- 

 dite (Fig. 516,16), and an inner one, the endopodite (Fig. 516,17). Although 

 this is the primitive condition of the appendage, the performance of special 

 functions has in many cases brought about a modification of it and often a 

 loss of some of its parts. 



* See "Crustacea," by A. Gerstaecker and A. E. Ortmann. Bronn's "Klass. u. 

 Ord. d. Thierreichs," Bd. 5, Abt. 1 and 11, 18G6-1901. "Crustacea," by J. S. Kingsley, 

 Standard Natural Hist., Vol. 2, 1888. "List of the Crustacea," by Mary J. Rathbun, 

 Fauna of New England in Occasional Papers of the Boston Soc. of Nat. Hist., VII, 

 1905, 



