CLASS CYCLOSTOMATA 419 



sac gives off ventrally a tube of unknown function, called the 

 hypophysis or pituitary body (Fig. 355, na'). 



The auditory organs of Petromyzon, which lie in the auditory 

 capsule (Fig. 354, 6), have only two semicircular canals instead 

 of the usual number, three (Fig. 350). The hagfish has only 

 one. The eyes of the lamprey are poorly developed. 



The Urinogenital System. - - The excretory and reproductive 

 systems are so closely united in the lamprey that it is custo- 

 mary to treat them together as the urinogenital system. The 

 kidneys lie along the dorsal wall of the body-cavity, and each 

 pours its secretions by means of a duct, called the ureter, into 

 the urinogenital sinus, and thence to the outside through the 

 urinogenital aperture. The sexes are separate, but eggs are some- 

 times present in the testis of the male. The single gonad (Fig. 

 355, ov) fills most of the abdominal cavity. The germ-cells 

 break out into the ccelom, make their way through two genital 

 pores into the urinogenital sinus, and then pass out through the 

 urinogenital aperture into the water, where fertilization occurs. 



Development. - - The eggs produce larvae known as Ammo- 

 ccetes. The larva differs in many respects from the adult, and 

 apparently represents a stage of development intermediate 

 between Amphioxus and a primitive vertebrate. As in Amphi- 

 oxus, food particles are drawn into the mouth by means of a 

 current of water produced by cilia. An endostyle, which repre- 

 sents the thyroid gland of the adult, secretes mucus which en- 

 tangles the food and carries it into the alimentary canal. 



The Ammoccetes lies buried in mud and sand, and probably 

 keeps its skin free from bacteria, fungi, and other parasitic 

 growths by means of an integumentary secretion. In the winter 

 of the third or fourth year the larval lamprey undergoes a meta- 

 morphosis during which the structure and habits of the adult 

 are acquired. 



Relationships. - - The hagfishes and lampreys are the lowest 

 vertebrates. Many of their structures, such as the cranium 

 and vertebral column, are very primitive, but others are appar- 



