THE PEIMITIVE VERTEBRATES. 



259 



in the Amphioxus, probably in consequence of secondary 

 reversion. (Cf. Chap. XIII.) 



Fig. 57. — Transverse section through the 

 anterior part of the primitive vertebrate type : 

 mr, spinal tube; x, chorda (notochord) ; msi, 

 dorsal muscles; kb, gill-vent ; k, gill-intestine. 



On both sides of the spinal tube 

 of all Vertebrates, and the notochord 

 which underlies it, great masses of 

 flesh are seen, which form the muscular 

 parts of the trunk and accomplish its movements. Although 

 in developed Vertebrates these masses are differentiated and 

 combined in various ways (corresponding to the variously 

 differentiated parts of the bony skeleton) yet in our ideal 

 primitive Vertebrate we can distinguish only two pairs of 

 main muscles which traverse the whole length of the body 

 parallel to the notochord. These are the upper, or dorsal, 

 and the lower, or ventral, side-muscles of the trunk. The 

 upper (dorsal) side-muscles of the trunk, the primitive 

 back-muscles (Fig. 58, msi) form the thick mass of the 

 flesh of the back. The lower (ventral) side-muscles, the 

 primitive abdominal muscles, on the other hand, form 

 the fleshy wall of the abdomen (Fig. 58, ms2). 



tnr 



171 ■s 



Fig. 58. — Transverse section through the 

 central portion of the ideal Primitive Verte- 

 brate : /, skin-fold, forming fin ; mr, spinal tube ; 

 a?, chorda; msi, dorsal muscles; msi, ventral 

 muscles ; a, aorta (in the mesentery) ; ma, 

 stomach-cavity ; c, body-cavity (visceral cavity) ; 

 hz, heart. 



Outside this wall we find the outer firm covering 

 of the whole body, called the leather-skin {corium, or 



