l6 COMPARATIXK ANATOMY 



Iniiulred l>ody sogiiionts, mcisl oi thciu nearly alike, and m«>sl bearing 

 one or two pairs of legs. The common myriapods, an inch or so in length, 

 are popularly supposed to he worms. 



The insects fairly rival the vertebrates as the dominant group in the 

 modern world. No vertebrate except man has carried social organizi\tion 

 nearly so far. No vertebrate groups and lew in\ertebrates at all approach 

 the insects in the number of individuals in a single species. In addition, 

 tlie number of species is enormous. In North America alone, there are 

 seven tliousand named species of butterllies and moths, ten thousand 

 beetles, and nearly as many tlies. Possibly, in the entire world, known 

 and unknown species together may number as many as a million. 



Fortunately, the individual insect is small. The largest known are a 

 beetle six inches long, and a grasshopper with a wing spread of ten inches. 

 But there are fossil dragontlies with a wing spread of two feet. From 

 these sizes, the insects run down to tiny parasitic creatures that live out 

 their life-cycle inside the speck-like egg of the coddling moth. 



We shall examine later the assumption that the two dominant groups, 

 the insects and the vertebrates, one of which contains the smallest of 

 highly organized metazoa and the other the largest, are genetically 

 related to one another. 



Phylum 12. Chordata 



The chordates are animals which, at least early in life, have a sup- 

 porting rod, the notochord or chorda dorsalis. between the alimentary 

 canal and the central nervous system. In lugher chordates the notochord 

 is replaced during ontogenesis by a cartilaginous or bony vertebral 

 column. All have a dorsal tubular nervous system. The heart is ventral, 

 and the pharynx has functional or embryonic gill slits. Most chordates 

 are metameric in structure, although the metamerism may become greatly 

 obscured in the adult. Segmental excretory organs are generally present. 



Neiirly ^o.ooo species are known. 



Four sub-phyla are included in the phylum. 



Sub-Phylum HE^^CHORDA i^Exteropxeusta) 



The hemichordates or enteropneusta hold a somewhat uncertain 

 position in the animal kingdom. Morphologists are by no means agreed 

 that their closest afhnities are wdth the chordates. Some associate them 

 with the annelids, while the resemblance of their larval stage to that of 

 echinoderms leacis others to place them near that group. Their inclusion 

 among the chordates rests on their possession of pharyngeal gill-slits, 

 enteric coelomic pouches, a notochord-like diverticulum of the fore-gut in 



