ORGANS OF CIRCULATION. 



199 



epithelium without muscular or adventitial envelopes, and at 

 times, as in the frogs, they expand into large subcutaneous 

 lacunar lymph spaces, or similar spaces around the gonads and 

 in the mesenteries, as in many ichthyopsida. The system is 

 frequently in connection with the caelom by means of openings 

 (stomata) in the peritoneal membrane. 



The distribution of these vessels varies greatly in the differ- 

 ent groups ; and a detailed comparative study of the system is 

 still a desideratum, while its development is largely unknown. 

 In the fishes there is a rich plexus of 

 lymph capillaries beneath the skin which 

 extends into the connective tissue be- 

 tween the muscles, while around the 

 heart and the ventral aorta the system 

 is richly developed. In the lower ver- 

 tebrates (amphibia, reptiles, and em- 

 bryo birds) pulsating sacs occur in the 

 course of these vessels, — the so-called 

 lymph hearts. These are usually placed 

 near some connection between the lymph 

 and venous systems, as near the pelvis 

 and the caudal vertebrae, or in the tho- 

 racic cavity dorsal to the heart ; but oc- 

 casionally lymph hearts occur at more 

 distant points. For instance, in the 

 urodeles a series of these occur beneath 

 the lateral line ; none are known in 

 mammals. 



In sauropsida and mammals a special 

 trunk, the thoracic duct, is developed in 



connection with the digestive tract which 



Fig. 209. Urogenital 

 system of tadpole of frog, 

 after Marshall and Bles. 

 A, radix aortae ; B, gills ; 

 F, fore foot ; OF, fat body ; 

 GL, glomus of head kid- 

 ney ; GR, genital ridge ; 

 H, heart ; M, mesone- 

 phros ; P, pronephric duct ; 

 PR, pronephros ; U, ureter. 



takes the lymph from the hinder extrem- 

 ities, the reproductive and excretory or- 

 gans as well as the alimentary canal, and carries it forward, pour- 

 ing it, in the sauropsida, into the right brachiocephalic vein, in 

 the mammals into the left.^ In birds and mammals valves 



1 According to the unpublished studies of Dr. F. D. Lambert, a paired thoracic duct is 

 developed in the young of Amblysioma, but a little later the right of these vessels ijecomes 

 obliterated. 



