HIGH SCHOOL ZOOLOGY. 47 



that gland, before it emerges through, the hepatic veins. The 

 venous blood from the head is returned more directly through 

 the antei'ior cardinal veins, which join the posterior cardinal 

 on each side before they enter the venous sinus. 



63. The lymph presents a contrast to the blood in this 

 respect, that it is not contained in well-defined vessels. 

 There are however a series of thin-walled channels, by which 

 the system is put in communication with the cardinal veins. 

 As for the lymph-glands referred to, the most important of these 

 is the spleen, a deep red body of considerable size near the 

 stomach, while the second almost equally as large and simiUi)- 

 in appearance, but very different in origin, is the head-kidney, 

 which lies between the anterior end of the air-bladder and tho 

 partition which walls off the pericardial cavity. 



64. Excretory System. — The structure of the head- 

 kidney is in the embryo catfish similar to that of the 

 kidney proper, which occupies the posterior part of the 

 ccelom, but in the course of growth the excretory tubes 

 which it possesses are replaced by lymphatic tissue, and 

 consequently it has no excretory function in the adult. On 

 the other hand the kidney proper which is separated from the 

 head kidney by the entire length of the air-bladder, is a true 

 excretory gland, which selects by filtration and otherwise from 

 the blood subjected to its action, certain nitrogenous excreta, 

 which have to be removed from the circulation. The excretion 

 is carried off from each half of the kidney, by a separate ureter, 

 the only indication here that the kidney is a paired structui-e, 

 and that consequently the right and left organs have coalesced. 

 The ureters join on leaving the kidney, and dilate into an urin- 

 ary bladder before opening exteriorly. 



65. Tliere is little external difference between the sexes in 

 the catfish, but there is one notable difference in habit which 

 ajjpears to be common to several allied forms. The eggs after 



