CRUSTACEA. 



415 



The males are smaller, sometimes even dwarfed, and then attached 

 to the females like parasites. They almost always possess appa- 

 ratuses for holding the females and for transferring the spermato- 

 phores during copulation. The larger females, on the other hand, 

 frequently carry the eggs about with them in sacs, the membranes 

 of which are secreted by the so-called cement glands. 



Development takes place either directly or by metamorphosis. 

 The metamorphosis is sometimes retrogressive. "When the develop- 

 ment is direct, the young animals, on leaving the egg, already have 

 the body form of the adult. The larva known as the Nauplius 

 (fig. 332) is of great importance as a point of departure. This 

 larva possesses an oval body, on the ventral side of which are present 

 three pairs of appendages for the sense of taste, the prehension of 

 food, and for locomotion. These appendages correspond to the two 

 pairs of antenme and mandibles respectively. Parthenogenesis is 

 said to occur in certain groups (Phyllo- 

 poda). 



Almost all Crustacea are carnivorous. 

 Some of them suck the juices of living 

 animals on which they are parasitic. 



For the systematic review of this 

 heterogeneous group, it is convenient to 

 divide the numerous orders into two 



FIG. 332. NanpHus larva of 

 Balanus, seen from the side. 

 A' First appendage (first an- 

 tenna); A!', second appendage 

 (second antenna) ; Mclf, third 

 appendage (mandible); Ob, 

 upper lip ; D, intestine. 



series. 



1. The small simply organized Crus- 

 tacea, the number and form, of whose 

 appendages is very various, will be in- 

 cluded as Entomostraca (0. Fr. Miiller). 

 To this group belong the orders Phyllo- 

 pcx.la, Ostracoda, Copepoda, and Cirripedta. 



2. The higher Crustacea, characterised by a definite number of 

 segments and appendages, may be grouped together as Malacostraca 

 (Aristotle). In this group are included the orders of Arthrostraca 

 (Ampldpoda and Isopoda), and Thoracostraca (Cumacea Slomaiopoda, 

 Scldzopoda, and Decajmda'). 



In addition there is the genus Nebalia, which has been hitherto 

 erroneously placed with the Phyllopoda, but which is to be regarded 

 as the representative of an ancient group connecting the Phyttopoda 

 with the Malacoslraca, and may be opposed to the latter as Lept- 

 ostraca. 



Finally, in addition to these chief divisions, there is a number 



