156 



PISCES. 



ment. In Myxine the urinary ducts open with the genital pore, in 

 Petromyzon into the intestine. In front of the kidneys, in the region 

 of the heart, there is another part of the kidney which in the adult 

 animal is no longer functional. This is the head-kidney or prone- 

 phros (Nebenniere of Joh. Miiller). It consists of a number of 

 glandular ducts, which begin with funnel-shaped openings into the 

 body cavity (pericardial cavity), and in the young animal open into 

 the urinary duct. 



The genital glands are unpaired in both sexes. In Myxine they 

 lie on the right side ; in Petromyzon in the middle line. They never 

 possess ducts, but the eggs and spermatozoa are at the breeding time 

 dehisced into the body cavity, whence they pass out through a pair 

 of genital pores placed behind the anus. 



The Petromyzontidce undergo a kind of metamorphosis, which was 



FIG. 602. a, Petromyzon Jtumatilis (after Heckel and Kner). I, c, d, stages in the trans- 

 formation of Ammoccetes branchialis into Petromyzon Phine-rl (after v. Siebold). b, Head 

 of an eyeless larva seen from the side ; c, the same seen from underneath ; d, later 

 stage with small eyes, seen from the side. 



discovered two hundred years ago by Baldner, a fisherman of Stras- 

 burg, but has only recently been rediscovered by Aug. Miiller. The 

 young larva? (fig. 602, b, c, d) are blind and without teeth. They 

 possess a small mouth, surrounded by a horseshoe-shaped upper lip. 

 and were for a long time placed in a special genus Ammoccetes. 



The Cyclostomes live partly in the sea; they ascend rivers at 

 spawning time, sometimes carried by the Salmon and Shad (Alausa 

 rl<jaris}, and deposit their eggs in holes in the river-bed. Others 

 are river-fish. They attach themselves to stones and to dead and 

 living fish, which latter they may in this way kill. They also eat 

 worms and small aquatic animals. The genus Myxine is exclusively 

 parasitic on other fish and even makes its way into their body cavity, 

 thus affording an example of an endoparasitic Vertebrate. 



Fam. Myxiuoidae (Hags). Head obliquely truncated ; suctorial mouth without 



