THE SKELETON OF VEKTEBRA.TES. 



29 



stated here that there are many animals belonging to this 

 branch which have the skeleton in the condition of cartilage. 



O ' 



as in the fishes known as sharks, skates, and sturgeons 



o 



(see Figs. 301, 294, 309), and others, as the fishes known 

 as the amphioxus, hag, etc. (see Figs. 351, 352), which have 

 no skeleton, not even a back-bone, nor any hard parts what- 

 ever. 



Therefore, to meet all cases, the Vertebrates may be de- 

 fined as animals having an elongated, bony, or cartilagi- 

 nous, or soft axis, with an elongated cavity or tube above 

 this axis, containing the brain and spinal cord, and called 

 the spinal cavity or neural canal, and a tube or elongated 

 cavity below this axis, containing the heart, the digestive 

 organs, and other organs of vegetative life, and some- 

 times called the haemal arch or hremal cavity from the Gr. 

 haima, blood.* So that a longitudinal, and a transverse 

 section of any vertebrate animal, the lowest as well as the 



FIG. 27 FIG. 28. 



h m 



A transverse, and a longitudinal section of a Vertebrate. 



a, alimentary canal ; A, heart ; c, cerebro-spinal center; s, sympathetic system ; n, 

 notochoril ; m, nioutli Huxley. 



highest, are represented by the accompanying diagrams. 

 In a word, all the Vertebrates are constructed according to 



* The cavity below the axis of the body, that is, the haemal cavity, corre- 

 sponds to the whole Interior of the body of the Invertebrate animals ; the cerebro- 

 spinal canal has no homologue, that is, it has nothing to represent it, in the 

 Invertebrata. 



