278 



CHORDATE ANATOMY 



Evolution of the Lymphatic System. Little is known of the relations 

 of chordate lymphatics to those of prechordates. The contrast between 

 lymphatics and blood-vessels is less marked in invertebrates than in 

 vertebrates. It seems not unreasonable to assume that primarily there 

 was no distinction between blood-vessels and lymphatics and that the 

 two systems have had a common origin. As in the case of most blood- 

 vessels it is impossible at the present time to homologize particular 

 lymphatic vessels in chordates and pre-chordates. 



In Amphioxus the lymphatics surround the blood-vessels and occur 

 also in the metapleural folds, dorsal fin, and around the central nervous 



Fig. 251. — The lymphatics of the scrotum. Showing the transition of the capillaries to 

 the vessels with valves (a, a, a). (From Morris, after Teichmann.) 



system. In vertebrates the distribution of lymphatics corresponds 

 roughly with that of veins, although lymphatics are far more variable in 

 position than are veins. Like the veins the lymphatics are divided into 

 superficial and deep systems. The deep system develops in close relation 

 to the cardinal veins and acquires connexions with them and with the 

 superficial system. 



Lymphatic vessels occur in cyclostomes and fishes. ' They surround 

 the veins in elasmobranchs, while their relations to the veins are less 

 intimate in other fishes. One or two main trunks may parallel the dorsal 

 aorta in this group and therefore are comparable to the paired thoracic 

 ducts of mammals. Lymph sinuses surround the heart and nervous 



