784 COMPARATIVE PHYSIOLOGY. 



into it the products of digestion are ultimately poured ; 

 from it waste products are filtered. It is itself, however, 

 confined to closed vessels, and does not come into close 

 connection with the tissues ; these are, strictly speaking, 

 nourished by the lymph, which bathes the tissues through- 

 out, and also communicates freely with the blood stream. 

 Thus the lymph is the " middleman " between blood and 

 tissues. In Vertebrates the lymph has not the respiratory 

 significance which the blood has in virtue of its red cor- 



o 



puscles. 



In most of the lower aquatic forms of life the fluid 

 within the body differs little from that which surrounds it. 

 Thus, as we should expect, the fluid which bathes the cavity 

 of a sea-anemone or a jelly-fish, filling the hollow tentacles 

 of the one and the canal system of the other, is little more 

 than sea water. It contains no formed elements, no dis- 

 solved albumens, no organic substances capable of forming 

 a loose combination with oxygen that is, no respiratory 

 pigment. It is thus certainly not a nutritive fluid ; the tissues 

 must be nourished by the products of digestion passing 

 from cell to cell. It is, however, of use in respiration. Like 

 other sea water, it contains dissolved oxygen; and we must, 

 suppose that the endoderm cells take up the oxygen they 

 require directly from it, as the ectoderm cells do from the 

 surrounding water. The fluid has also an excretory signi- 

 ficance : it carries away waste products, both solid and 

 gaseous, and removes these from the body. 



The fluids of Ascidians, Lamellibranchs, and of a few 

 Gasteropods, are all classed by Krukenberg as hydrolymph. 

 They consist largely of water, but contain in addition formed 

 elements, or dissolved proteids. In Ascidians the body 

 fluid contains a small amount of dissolved proteids, and 

 some pigmented corpuscles. 



In Echinoderms we find that both a perivisceral fluid 

 and blood enclosed in special blood vessels are present. 

 Of the blood little or nothing is known, the technical 

 difficulties in the way of isolation being very great. The 

 perivisceral fluid contains numerous formed elements, and 

 a small amount of dissolved proteids. It probably performs 

 the functions of the lymph of Vertebrates, but is said to 

 have a respiratory function in addition. 



