544 Animal Biology 



aquatic life at certain periods of their life history. Many Crustacea 

 (subclass Copepoda) (Fig. 127) are fish parasites. 



Barnacles are degenerate Crustacea (subclass Cirripedia) which en- 

 crust the bottoms and sides of ships, wharves, and piles, and are an 

 annoyance to bathers, while other species act as parasites (Fig. 133). 



The "sow bugs" or "wood lice" (subclass Malacostraca; order Isop- 

 oda) are grayish Crustacea found in dark, moist places, usually under 

 boards and rocks. They breathe by means of abdominal gills and feed 

 on decaying vegetable matter, although they may attack living plants 

 (Fig. 126). 



Several species of shrimp (subclass Malacostraca; order Decapoda) 

 are found along our coasts and are widely used for human food. 



Several species of crab (subclass Malacostraca; order Decapoda) such 

 as the blue crab or soft-shell crab, the painted crab, the rock crab and 

 the oyster crab are commonly used for human food (Fig. 132). 



In general, the Crustacea are most cosmopolitan in their geographic 

 distribution, thus ensuring their existence and consequently being either 

 detrimental or beneficial. They also produce large numbers of offspring, 

 which naturally affects their economic importance. Several deep-sea 

 Crustacea are phosphorescent and many have brilliant colors. 



Class Diplopoda and Class Chilopoda 



Very few of the millipedes (Fig. 135) are of economic importance. 

 The common house centipede feeds on bedbugs, flies, and cockroaches 

 (Fig. 135, C) . It is not very poisonous to man. The venom of the large 

 tropical centipedes may be fatal to man and other animals. 



Class Arachnoidea 



The "red spider," which is a mite, attacks nearly two hundred dif- 

 ferent plants, especially in greenhouses (Fig. 270). 



The follicle mite (Demodex folliculorum) produces "blackheads" in 

 man and other mammals by entering the hair follicles. The itch mite 

 parasitizes the skin. The so-called chigger (the young of the harvest 

 mite) burrows into the skin of man and other mammals, causing a 

 severe irritation (Fig. 269). The common tick transmits the organisms 

 which cause the disease of African relapsing fever (Fig. 138). Ticks 

 and mites are rather small, being external or ectoparasites in many 

 instances. Some forms burrow beneath the skin, causing rather severe 

 irritations, while others merely suck blood from the host. Some types 

 are able to transmit the causes of diseases from one host to another. An 



