438 ARTHROPODA 



55. The Neuroptera have net-veined wings and a holometabolous 

 development; the mouth parts are modified. 



56. The Coleoptera are biting insects with the fore wings changed 

 to elytra; they differ from the somewhat similar Orthoptera by the com- 

 plete metamorphosis. 



57. The Strepsiptera are parasitic forms allied to the Coleoptera. 



58. The Hymenoptera have partly biting, partly licking mouth 

 parts; membranous wings with few nervures and holometabolous 

 development. 



59. The Rhynchota are hemimetabolous or ametabolous, with 

 piercing mouth parts; the bed bugs and the Pediculina are parasitic. 



60. The Diptera are holometabolous, with piercing mouth parts and 

 not more than one pair of wings. The larvae of the CEstridoe are parasitic. 



61. The Aphaniptera are holometabolous, wingless, parasitic, with 

 sucking mouth parts. 



62. The Lepidoptera have the wings covered with scales; labium and 

 labrum rudimentary, the maxilke altered to a sucking tube; the develop- 

 ment holometabolous. 



63. The DIPLOPODA have a head with three pairs of appendages; 

 the trunk with double segments, each bearing two pairs of legs, the genital 

 openings anterior. 



64. The term Myriapoda is frequently used to include Chilopoda and 

 Diplopoda. 



PHYLUM VIII. CHORDATA. 



Within recent >ears it has been realized that a number of animals, 

 formerly distributed among various groups, possess structural features of 

 great importance which ally them to the vertebrates; but they lack the ver- 

 tebrae and many other features characteristic of that group, so that the name 

 cannot be extended to embrace them. Yet since all possess, as a temporary 

 or a permanent feature, a structure known as the chorda dorsalis or noto- 

 chord, the term Chordata has been adopted to include them. The notoclwrd 

 is an elastic rod arising from the entoderm and coming to lie between the 

 digestive tract and the nervous system (fig. 9). 



In all Chordates the anterior (pharyngeal) portion of the alimentary 

 canal develops several pairs of pockets which grow outwards and fuse 

 with the ectoderm. The fused portion then breaks through, and the 

 pockets become converted into gill slits (branchial clefts), which, in the 

 lower forms, allow the passage of water over the gills which line them. 



The central nervous system lies on one side of the alimentary canal,, 



