CIIORDATA. 501 



parts; membranons wings with few nervures and holometabolous 

 development. 



5'.l. Tlie Rhijnchota are hemimetabolons or anietabolous, with 

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



60. The Diptera are holometabolous, with jjiercing mouth parts 

 and not more than one pair of wings. The larva of the (Estridas 

 are parasitic. 



01. The Ajjlianiptera are holometabolous, wingless, parasitic, 

 with sucking mouth parts. 



02. The LepidojAera have the wings covered with scales ; labium 

 and labrum rudimentary, the maxilla altered to a sucking tube; 

 the development holometabolons. 



Oo. The DiPLOPODA have a head v.'ith three pairs of appendages ; 

 the trunk with doul^le segments, each bearing two pairs of legs, 

 the genital openings anterior. 



0-i. The term Myriapoda is frequently used to include Chilop- 

 oda and Diplopoda. 



PHYLUM VIII. CnOEDATA. 



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 vertebra and many other features 

 characteristic of that grouji, 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 alimentar}^ canal develoj^s 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), wliich, in 

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

 line the slits. 



The central nervous S3'stem lies on one side of the alimentary 

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

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

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

 bodv around tlie blastopore. The edges of this plate are rolled 



