SIDE CANALS AND BODY-CAVITY. 423 



we conceive that this important resemblance is more than 

 a mere analogy. It has the deeper significance of a true 

 homology, and rests on a morphological resemblance of the 

 organs compared. Thus, the Amj^hioxus shows us that the 

 aorta, the single main artery of Vertebrates, running 

 between the intestine and the notochord, represents the 

 dorsal blood-vessel of Worms. On the other hand, the ven- 

 tral blood-vessel of the latter is retained only in the single 

 intestinal vein passing below the intestine of the Amphioxus 

 (and its anterior continuation ; cystic vein, liver vein, 

 hollow vein (y. cava), gill-artery). In the developed body 

 of all other Vertebrates this intestinal vein (originally the 

 main venous blood-vessel !) is far outstripped by other 

 veins. 



Together with the real blood-vessels, special absorbing 

 lymph-vessels seem to exist in the Amphioxus, Several 

 canals, extending under the skin, have recently been 

 regarded in this light, especially th^ narrow "ventral canals" 

 (Fig. 152, S-^J, and wide "side canals" (S). Both pass along 

 the whole length of the ventral side and contain colourless 

 lymph. The side canals (S) must possibly be regarded as 

 the last remnants of degenerated primitive kidney ducts. 

 They lie in the two parallel side folds of the ventral skin 

 (F), ending blindly both in front and behind, and do not 

 open outwards, as was supposed till recently. 



The real body-cavity {codoma) in the Amphioxus (Fig. 

 152, Lh) is extraordinarily narrow and small. It surrounds 

 the intestinal tube in its narrow cavity, and is probably con- 

 nected with the lymph spaces. Formerly it was confused 

 with the large respiratory cavity or gill-cavity {A), which is 

 of entirely different morphological and physiological signifi- 



