CHORDATA 201 



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

 cavity serves as a digestive tract. In the earthworm the 

 cavity, or coelome, as it is properly called, contains blood 

 vessels, nerve cords and ganglia, and an ahmentary canal 

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

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

 most of the members ^^^ 



of the Chor data, and " 'M^^^l 



especially in the ver- ^^ ^-T>^....j n.-—j^o «^^^ 



tebrates, we find two t ^^^^ \ f ^^^^^ \ 



(Fig. 142, 7). One of V ^"^ J V ^^ J 

 these cavities contains ^ ^^—-<^:^ -—-n. ft---\ ^"Q ^^ r 



the blood vascular / v 



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



nerve gangha, and bodyof an invertebrate (/), and a-vertebrate 



'^ ^ ' (y). Note the one body cavity in 7, and the 



the alimentary canal. two body cavities in y : a, aUmentary canal ; 



This CavitV COrre- ^' b'°°'i vascular system ; n, nervous system ; 



, . , , , c, spinal cord. 



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 

 aU the invertebrates. 



Note that the two cavities in the Chordata are separated 

 either by a notochord or by a series of bony 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 



