114 ELEMENTS OF BIOLOGY 



Class IIL MYRiAPODA (Fig. 59). Centipedes and Millipedes. 



Class IV. iNSECTA (Fig. 60). Body divided into head, thorax, 

 and ABDOMEN. Always three pairs of walking legs attached to the 

 thorax. Breathe by means of trachese. Nineteen orders of Insect 

 are recognized, many of which have the power of flight. Because 

 of their highly specialized tissues, their extraordinary display of 

 instincts, their tendency to assume social life, their ready adapta- 

 bility, and other characters, some authorities regard the Insects as 

 equal to the Mammals in complexity (Fig. 73). 



Class V. arachnida (Fig. 61). The Spiders. Four pairs of walk- 

 ing legs attached to the thorax. Never have wings. 



Phylum CJwrdata 



Members of this phylum are characterized by the following com- 

 mon features: At some time during their life history there is pres- 

 ent a longitudinal rod or axis dorsal to or above the alimentary 

 canal, termed the notochord (Fig. 64) ; the main portion of the 

 nervous system consists of a tube of nervous tissue dorsal to the 

 notochord and hence dorsal to the digestive tube; the anterior por- 

 tion of the animal is perforated by paired slits which connect the 

 forward region of the digestive tube with the outside. These slits are 

 always present in the embryo but in many forms are widely changed 

 in the adult. Chordates are commonly divided into four sub-phyla: 



Sub-phylum I. enteropneusta (Fig. 62). Worm-like animals not 

 very clearly related to other chordates. 



Sub-phylum II. tunicata (Fig. 6^). The Sea Squirts. The adults 

 of this group appear to be degenerate types, for the larvae are free 

 swimming forms outwardly resembling small frog tadpoles, while 

 the adults are attached forms in which the chordate characters have 

 become somewhat obscured. 



Sub-phylum IIL cephalochordata (Fig. 64). The Lancets. These 

 are small forms shaped like fishes; in some sections of the world 

 they are widely used for food. The form is of peculiar interest to 



