324 INTRODUCTION TO ARTHROPODS 



so conspicuous in the annelids, has been entirely usurped by a great 

 expansion and running together of bloodvessels so that in the 

 place of the usual body cavity or crelom there is a large blood- 

 filled space. Within this space are bloodvessels and a so-called 

 heart, which retained their individuality while the other vessels 

 fused. 



Classification. The phylum Arthropoda is divided into five 

 classes. One of these, the Onychophora, includes only a single 

 genus of animals, Peripatus, which is very primitive, and helps 

 to bridge the gap between the more typical arthropods and the 

 annelids. Peripatus is a free-living wormlike animal and of no 

 interest here. The remaining four classes are the Crustacea, 

 Arachnida, Myriapoda and Insecta. 



The Crustacea, including crayfish, water fleas, etc., are pri- 

 marily arthropods of the water. They are geologically of great 

 antiquity and among them are the most primitive of the typical 

 arthropods. Their appendages are usually numerous and, taking 

 the group as a whole, show a wonderful range of modifications 

 for nearly every possible function. Crustaceans breathe by gills. 

 Although many are parasites of aquatic animals, none can be 

 considered as parasites of man or other land animals. In two 

 cases Crustacea are known to serve as the intermediate hosts of 

 human parasites, namely Cyclops as a host for the guinea-worm 

 (see p. 312), and the Japanese land crabs as the second inter- 

 mediate hosts of the lung fluke (see p. 222). 



The Arachnida, including spiders, scorpions, mites, etc., are 

 for the most part highly developed arthropods, representing the 

 terminus of a separate line of evolution. They probably had a 

 common origin with the Crustacea, but have become adapted 

 to terrestrial life. The members of this class have four pairs of 

 legs as adults, two pairs of mouthparts and no antennae. They 

 breathe by means of invaginations of the body which contain 

 gills arranged like the leaves of a book, whence the name " book 

 lungs." Some of the higher arachnids also have a system of 

 branched air tubes or tracheae in the body similar to those found 

 in the insects and myriapods. Two orders of Arachnida contain 

 parasitic species, namely the Acarina, or mites and ticks, and 

 Linguatulina, or tongue-worms. Many ticks are disease carriers. 



The Myriapoda, including centipedes and millipedes, are 

 terrestrial arthropods which breathe by tracheae. The body is 



