GENERAL ZOOLOGY 



defined bv segmentation, the exoskeleton, and the jointed appendages. No 

 other phylum of animals approaches the arthropods in numbers of species. It 

 has been estimated that the number of species of insects alone is well over 

 1,000,000, as compared with a total of some 37,500 species of chordates and 

 60,000 species of moUusks. If we add to the insects an estimated 30,000 

 species of crustaceans, 16,000 species of chelicerates, and 2000 species of chilo- 

 pods and diplopods, the total is much larger than that of all other species of 

 animals. We are accustomed to think of vertebrates as the dominant forms of 

 terrestrial life at the present day; the vertebrates are far exceeded in numbers 

 of species and numbers of individuals by the insects, which are so numerous 

 that they literally contend with the vertebrates for possession of the earth. 



The arthropods are of considerable economic importance. Crustacea, such 

 as lobsters, crabs, and shrimps, are a significant source of food. Certain in- 

 sects produce silk, others prepare honey, and through the agency of insects 

 many plants useful to man are pollinated. Some insects are beneficial to man 

 in that they prey upon, or parasitize, other kinds of insects. On the other 

 hand, the arthropods include species that destroy almost every form of vegeta- 

 tion, others that are parasitic on man and his domestic animals, and still 

 others that transmit the organisms of disease to man. 



The oldest known fossil remains of arthropods are those of trilobites (Fig. 

 15.1); according to the fossil record, these forms were already abundant and 

 highly specialized in Cambrian times. It is logical to assume that these com- 

 plex forms arose through a long Pre-Cambrian evolutionary history, and 

 hence that more primitive and generalized arthropod types must have been in 

 existence, possibly for some millions of years, before the beginning of our fossil 

 record. All the modern arthropod groups had ari.sen and become well di- 

 versified by the early part of the Mesozoic Era, roughly 200,000,000 years 

 ago (see Fig. 20.1, p. 617). By this time trilobites had become extinct. 



Although Chelicerata undoubtedly arose earlier than Mandibulata, the two 

 most successful and significant modern groups of arthropods are the mandi- 

 bulate forms included in the classes Crustacea and Insecta. In the discussion 

 to follow, we shall devote our attention chiefly to these two classes, referring 

 more briefly to other arthropods. 



THE CLASS CRUSTACEA 



The Crustacea, for the most part aquatic animals, include the principal 

 marine representatives of the phylum Arthropoda. During their evolution 

 the crustaceans appear to have spread from their primitive habitat, the ocean, 

 to fresh water and in a few cases to terrestrial life, in a manner reminiscent 

 of the gastropod mollusks. The two principal types of crustaceans, Entomos- 

 traca and Malacostraca, will be considered after we have examined the cray- 

 fish as a typical crustacean. 



The Crayfish: Habitat and Activities. Crayfishes are abundant in 

 streams and fresh-water ponds of all the continents and many of the larger 



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