200 ZOOLOGY. 



rest of the intestine is quite straight, and cannot be divided 

 into ileum and rectum, nor does it end in a cloaca. 



9. The Circulation is very simple, there being no dis- 

 tinction of arteries from veins, and very little between these 

 and capillaries. Yet its identity of fundamental plan with 

 the dogfish's is plain when we allow for the absence of the 

 cardinal veins. The ventral or cardiac aorta lies in the 

 floor of the pharynx, under the endostyle, and sends branches 

 up along the primary gill- arches. There is no heart, but 

 the whole of the cardiac aorta is contractile, and at the 

 bases of the aortic arches that run up the gill arches there 

 are contractile dilatations that assist in the propulsion of 

 the blood. Branches from the vessels in the primary arches 

 run across the transverse bars into the secondary arches and 

 up these. Just above the gill-slits these vessels open into a 

 network of small vessels (glomus) which retards the flow 

 of blood through this region : the object of this we shall see 

 presently ( 13). All these branchial vessels end dorsally 

 in a pair of dorsal aortae, lying on either side of the hyper- 

 pharyngeal groove. Where this groove ends, at the be- 

 ginning of the " stomach," these aortas unite into a median 

 dorsal aorta from which branches are distributed over the 

 intestine walls, to unite on the ventral side again into a 

 sub-intestinal vessel, which corresponds to a portal vein, 

 as it takes the blood back from the intestine to the liver. 

 Thence hepatic veins take it to the ventral aorta, and so 

 the circulation is complete. 



It will be noticed that there is apparently no blood-supply 

 to the myotomes. These, however, and other parts of the 

 body are plentifully supplied with lymph-space?. Of these, 

 the metapleural canals in the metapleural folds are most 

 conspicuous. Little is known as to the course of the lymph 

 or its relation to the blood. The blood is colourless and 

 contains colourless corpuscles only. 



10. The Skeleton of Amphioxus is far simpler than 

 those of our other types. There are no vertebrae and no 

 skull. Instead, we have a longitudinal rod, the notochord, 

 which has already been mentioned (chap, vi., 15), as an 



