CHORDATA. 501 



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 Dipt era are holometabolous, with piercing mouth parts 

 and not more than one pair of wings. The larvae of the (Estridae 

 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 maxillae altered to a sucking tube; 

 the development 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 Chilop- 

 oda and Diplopoda. 



PHYLUM VIII. CHOKDATA. 



Within recent years it has been realized that a number of ani- 

 mals, formerly distributed among various groups, possess structural 

 features of great importance which ally them to the vertebrates. 

 On the other hand they lack the vertebrae and many other features 

 characteristic of that ^roup, so that the name cannot be extended 

 to include them. Yet since all these forms possess, as a temporary 

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

 or notochord, the term Chordata has been adopted to include them. 



The notchord is a smooth elastic rod arising, in development, 

 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 one or 

 more pairs of pockets which grow outwards and fuse with the 

 ectoderm. The fused portion then breaks through, and the pock- 

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

 the lower forms, allow the passage of water over the gills which 

 liue the slits. 



The central nervous system lies on one side of the alimentary 

 canal, there being no ring of nervous matter (Enteropneusta ex- 

 cepted) around the oesophagus, such as is so common in the in- 

 vert ebrata. It arises as a medullary plate on the dorsal side of the 

 body around the blastopore. The edges of this plate are rolled 



