STRUCTURE OF THE VERTEBRATES 19 



with the notochordal sheaths. Each fiber is attached at either end 

 to the myoseptum, so that the myotomes are functionally con- 

 tinuous although broken into definite metameres. 



Any muscular contraction exerts a pull on the anterior-pos- 

 terior axis, so that the range of movement is limited to a side 

 to side motion. Forward movement of the animal is caused by 

 the larger size of the anterior end. The tail region is more 

 flexible, and an alternate contraction of the muscles of either 

 side causes the tail to act as a propeller both in swimming 

 and in burrowing. 



Vascular System. The blood system of Amphioxus is the per- 

 fect starting point for a study of the vertebrate system. A glance 

 at the diagrammatic drawing will assist in understanding the 

 description. Recall that the vascular system has the functions 

 of (1) taking up absorbed food materials from the digestive 

 tract; (2) carrying waste products from the cells to the organs 

 of excretion (and respiration) ; (3) taking up oxygen from the 

 gills; and (4) distributing food and oxygen to the tissues of the 

 entire body. Further, Amphioxus, like the vertebrates, has a 

 closed system. The blood vessels are continuous, and the blood 

 remains within the lining of the vessels; therefore, when an 

 artery divides into capillaries, these same capillaries re-collect 

 into veins without any definite break. Consequently it is impos- 

 sible to make any definite distinction between arterial and 

 venous capillaries. 



Amphioxus has a slightly enlarged, pulsating ventral aorta, 

 homologous with the vertebrate heart. From this point the aorta 

 continues anteriorly, giving off a pair of afferent branchial 

 arteries at alternate gill supports. These arteries break into 

 smaller vessels in the gills, and then collect into efferent bran- 

 chial arteries. Lateral branches pass into the secondary gill 

 supports, with the result that there is an efferent artery for 

 each gill. Passing through the gills the blood has been aerated. 

 The efferent branchials enter the paired dorsal aortae, one on 

 either side of the notochord. Branches go forward into the head 

 region, the two dorsal aortae extending backward and fusing 

 to form a single vessel at the posterior end of the pharynx. The 

 aorta then continues al^Aft- the body and into the tail, giving off 

 metameric vessels to the muscles and the digestive tract. 



