PHYLUM ARTHROPOD A 



217 



The antennal glands are the typical excretory organs of adults, 

 though larvae and some Isopoda have a shell gland communi- 

 cating with the second maxillae for excretion. The genital 

 opening of the male is characteristically upon the coxopodite of 

 the eighth thoracic appendage and of the female on the sixth. 



Within the Malacostraca, a considerable number of orders are 

 recognized, but for the purposes of this text attention is limited 

 to but six of them. 



I. Order Phyllocarida 



A few marine species belonging to the genus Nebalia (Fig. 101) 

 show unusual combinations of phyllopodan and malacostracan 



Fig. 101. — Male of the genus Nebalia. {AfUr Claus). 



characters. In number of somites and in location of the genital 

 openings, they agree with the higher Crustacea, yet the thoracic 

 appendages are leaflike. 



II. Order Schizopoda 



These small, mostly marine, Malacostraca with compound eyes 

 on movable stalks bear a delicate carapace covering the cephalo- 

 thorax. The eight appendages of the thorax are biramous 

 swimming organs with both exopodite and endopodite and in one 

 family (Mysididae) bear gills projecting freely into the water. A 

 postabdominal somite bears appendages which with the telson 

 form a caudal fin. The use of this fin causes the animal to swim 

 backward as do crayfishes and lobsters. This order includes a 

 number of pelagic marine forms and the family Mysididae some 

 species of which inhabit fresh water. Mysis relicta occurs in the 



