CHORDATA 



201 



sea anemone, earthworm, and squid. In the hydra this 

 cavity serves as a digestive tract. In the earthworm the 

 cavity, or ca?lome, as it is properly called, contains blood 

 vessels, nerve cords and gangUa, and an alimentary canal 

 (Fig. 142, /). Then we may say, in general, that inverte- 

 brates have one cavity m the body. On the other hand, in 

 most of the nieml:)ers 

 of the Chordata, and 

 especiaUy m the ver- 

 tebrates, we find two 

 cavities in the body 

 (Fig. 142, 7). One of 

 these cavities contains "^ ^.-..^.^.X" . * 

 the blood vascular / 



system, a series of Fig. 142. — Diagrammatic cross sections of the 

 nerve e'ano'lia and bo<Jy of an invertebrate (/), and a vertebrate 



the ahmentary canal. 

 This cavity corre- 

 sponds quite closely 

 to the one cavity found in the bodies of the inverte- 

 brates. The other cavity in the Chordata contains the 

 brain and spinal cord, and we find nothing among the in- 

 vertebrates to correspond with this cavity. It is well to 

 bear in mind that the possession, by the Chordata, of a 

 second cavity containing the brain and spinal cord consti- 

 tutes a well-marked difference between this branch and 

 all the invertebrates. 



Note that the two cavities in the Chordata are separated 

 either by a notochord or by a series of JDony segments that 

 make up the spinal column. Note also that in whatever 

 animal a spinal column is found it is always preceded by a 

 notochord in the embryonic stages of that animal. With 



(V). Note tlie one body cavity in I, and the 

 two body cavities in V : a, alimentary canal ; 



b, blood vascular system ; ?i, nervoiLs system ; 



c, spinal cord. 



