332 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



trunks a cephalic trunk on either side empties. More superficial is a trunk on 

 either side which follows the lateral line, and connected with this are a series 

 of lymph hearts, from fourteen to twenty in number. In the anura (frog) is 

 the replacement of the ordinary tubes by large spaces, some superficial, some 

 deep in the body, into which the lymph capillaries empty. These are connected 

 with each other, while they are connected with the veins by openings into the 

 vertebral branch of the internal jugular and into the iliac, there being lymph 

 hearts connected with each of these openings. The anterior lymph hearts lie 

 between the transverse processes of the third and fourth vertebrae, the caudal 

 lateral to the end of the urostyle. In the anuran tadpole there are additional 

 hearts in the trunk which follows the lateral line. The caecilians are stated to 

 have a pair of lymph hearts for each segment of the trunk. 



Fig. 357. — Early lymph system (black) of 10 mm. rabbit embryo, after F. T. 

 Lewis, at, anterior tibial; c, caudal; /*, primitive fibular; ei, ij, external and internal 

 jugular; em, external mammary; pc, postcardinal; ul, primitive ulnar veins. 



In the reptiles there is a well-developed subvertebral lymph trunk, which is 

 said in some cases to envelope the adjacent blood-vessels, dividing in front to 

 empty into both brachiocephalic veins. Behind there is a connexion with the 

 ischiadic and renal veins. Near this point, usually just posterior to the dorsal 

 end of the ilium, there is a lymph heart on either side. The vessels are numerous 

 and very large in snakes, and a pair of lymph hearts occur in the same relative 

 position as in other orders. In the birds there are paired main vessels (thoracic 

 ducts) united behind, and there are two pelvic hearts, which, however, are not 

 contractile in the adult ratites and disappear from the adults of other groups. 



In the mammals the primitively paired thoracic ducts are sometimes re- 



