CRUSTACEA 



91 



antennules are also provided with olfactory hairs. Many of 

 this order are marine. Some, however, live in brackish or in 

 fresh water. They live usually at the bottom of their aquatic 

 habitat. 



Order III. Copep'oda. — As examples may be named para- 

 sitic fish lice and the fresh-water cyclops. Respiration takes 

 place over the entire body surface. 



The Cyclops (Fig. 65) is a small, 

 white, shelless animal with elongated 

 segmented body. It has a rather large 

 eye in the center of its head. 



Order IV. Cirripe'dia or Barnacles. 

 — These fixed, marine, shelled crusta- 

 ceans are very abundant along the 

 seacoast, the rocks being covered with 

 them in places. Their food consists 

 of small animals in the water. One 

 may see thousands of barnacles snap- 

 ping their food as the waves and tides 

 dash over them. 



Some forms attach themselves to 

 crabs, moUusks (Fig. 52), or even to 

 whales, while others are true external 

 parasites, sucking the juices of the ani- 

 mals to which they are attached. The 

 parasitic forms are extremely degenerate. 



Since they have no power of loco- 

 motion by which to escape their ene- 

 mies, the barnacles (Fig. 66) are pro- 

 tected by shells capable of " complete 

 closure." The body is flexed ventrally 

 and bears six pairs of cirri, which are used in straining small 

 organisms from the water and in carrying them to the mouth. 

 The mouth is surrounded by a pair of mandibles and two pairs 

 of maxillae. Barnacles are hermaphroditic, but cross-fertiliza- 

 tion may occur. They have a metamorphosis, having first a 

 nauplius and then a cypris stage, the latter developing into 

 the fixed adult (Fig. 67). This order furnishes a good illus- 

 tration of the principle that inactivity leads to degeneration. 



Fig. 65. — Cyclops: e, 

 Eye; h, heart; eg, feet; 

 /, eggs. (Clark.) 



