THE PHYLUM ARTHROPODA 3 



The foregoing paragraphs will explain the point of view from which 

 this book has been written, and the arrangement of the succeeding chap- 

 ters. The second chapter is devoted to the anatomy of the Diptera, 

 the subject being dealt with at some length, in order that it may serve as 

 an introduction to the anatomy of the other groups of arthropods. The 

 first section, dealing with the external anatomy, includes also an account 

 of those characters on which the accepted systems of classification are 

 built. In the following chapters each group of arthropods which is 

 concerned with the transmission of disease is dealt with separately, as 

 regards classification, anatomy, and technique, etc., a list of the principal 

 papers dealing with the group being given at the end of the chapter. For 

 further references the lists given at the end of these papers should be 

 consulted. 



Some knowledge on the part of the reader of the diverse modes in 

 which the arthropods may affect man and the domestic animals may be 

 safely assumed, and it will not be necessary to enter into a general discus- 

 sion of such matters here ; the economic importance of the various groups 

 will be discussed as occasion arises, so far as it falls within the scope of a 

 book of this nature. There are, however, certain points regarding the 

 zoological position of the blood-sucking forms which call for a brief 

 mention. 



The phylum ARTHROPODA is one of the principal divisions into 

 which the METAZOA, or many celled animals, are divided. It is an 

 extremely large one, and contains an enormous variety 



of forms. According to Shipley, at least seven-eighths Zoo'ogical position 

 , . . J c c 4 , ,, . of the blood-sucking 



of the protoplasm on the surface of the world is Arthropoda 



contained within the skins of arthropods, while in 



size and outward form they vary from the lobster to the most minute 



midge. 



Briefly, the essential characters of the phylum are metameric segmen- 

 tation, the presence of jointed appendages, typically one pair to each 

 segment, and a more or less hard exo-skeleton ; the heart is dorsal, in 

 position, and the nervous system ventral, except in the head, where a 

 part of the brain is dorsal ; the body cavity is a haematocoele. The 

 phylum is divided into five classes, as follows : 



CRUSTACEA : Crabs and lobsters, shrimps, cyclops. 



PROTRACHEATA : Peripetus. 



MYRIAPODA : Centipedes and millipedes. 



INSECTA : Insects. (Sometimes called hexapods). 



ARACHNIDA : Spiders, ticks, mites. 



