336 THE CP.AVII.SU AND THE ARTHROPUDA 



highly specialized. There are five principal types, as represented 

 by the Orders Phyllopoda, Cladocera, Ostracoda, Copepoda, and 

 Cirripedia. Like the Malacostraca, the vast majority of these 

 are marine, although the Phyllopoda and Cladocera are for the 

 most part fresh-water animals. 



Among the Phyllopoda, which are otherwise known as the 

 " fairy shrimps," species of the genus Branckineda are familiar 

 representatives (Fig. 162). The animals swim upon their backs 

 by means of their paddle-like thoracic appendages. The typical 

 habitat is in the shallow pools of fresh water that are formed 

 by the spring rains and become dry later in the season. Under 

 such conditions the organism must be able to withstand rather 

 rapid changes in the salt content of the water. This probably 

 explains the habits of the phyllopod called the " brine shrimp," 

 Artemia, which lives in salt lakes and in the evaporating basins of 

 salt works, where the salinity far exceeds that of the ocean. Yet, 

 strange to say, none of the fairy shrimps are marine. The males 

 of Branchinecta are distinguishable from the females by their 

 second antennse, which are modified as organs by which the 

 males clasp the females at the time of sexual union. The females 

 have a brood pouch or uterus, on the ventral side of the abdomen, 

 in which the eggs are carried for a time. After the eggs become 

 free they can withstand prolonged dessication if the pools become 

 dry. They are often carried for long distances by winds or 

 by the mud adhering to the feet and legs of vertebrate animals. 

 Eventually, the young emerge as free-swimming larvae, with 

 three pairs of appendages and a median eye. In this stage 

 they are known as nauplii and are comparable with the nauplius 

 stage that appears in the development of many higher Crustacea 

 such as the crayfish (c/. Fig. 161 A). The adult arises from 

 this nauplius by a series of molts, during which the characteristic 

 features are gradually acquired. 



The Crustacean Plan of Body. — We are now in a position to 

 consider the general organization of the crustacean body. The 

 simplest types of Crustacea are evidently forms in which the 

 metamerism is recognizable externally, not only in the paired 

 appendages but also in the well-marked constrictions between 

 the metameres on the dorsal and lateral parts of the thorax and 

 abdomen. A form like Branchinecta will serve to illustrate this 

 condition. The head is always a single mass externally, but the 



