348 BIOLOGY. 



numerous holes or pores. Here they absorb air, there 

 water. 



1948. Every absorbent point of integument is drawn 

 out as a tube towards the respiratory system, in order to 

 let what it has absorbed become oxydized. These tegu- 

 mentary prolongations into tubes are the lymphatic vessels. 



1949. In all teguments there are necessarily lym- 

 phatic vessels, but more of them in the water- than the air- 

 teguments. On that account the lymphatic vessels are 

 much more numerous in the intestine than the skin. 

 They are there called chyliferous, or lacteal vessels. 



1950. The lymphatic vessels are the first of all vessels. 

 Many animals, as perhaps the Acalephse and Distomata, 

 appear to have only this kind of vessel. 



1951. Lymphatic vessels are present in the skin, only 

 in so far as it has resigned the respiratory function to 

 special organs. 



Meaning of the Unclosed Vascular System. 



1952. The action of this unclosed vascular system 

 is wholly similar to the motion of sap and air in the 

 plant, there being only an ascent of the first, and a fall or 

 descent of the last. In the absorbents the sap ascends 

 out of the root (intestine) into the leaves (branchiae) ; in 

 the respiratory vessels the air descends from the foliage 

 (skin) to the intestine and the whole body of cells. 



1953. This vascular system is therefore the pure 

 remnant of the plant, and has as yet assumed no properly 

 animal character, except that both its sets of vessels 

 or tubes are self-substantial, and ramify, while in the 

 plant they are only inter- cellular passages or non-ramified 

 spiral vessels. 



1954. In insects this system has been most perfectly 

 evolved; there the air-tubes, being well parcelled out, 

 are in great number, and run directly to the intestine 

 and dorsal vessel, which is, as it were, only the trunk of 

 the lymphatic vessels, or the chyliferous duct. 



1955. Now, such a vascular system, merely oscillating, 

 as it were, between intestine and skin, can be but per- 



