CYCLOSTOMATA. 



99 



becomes established behind the pronephros, and in an Ammoccetes of 

 65 mm. the pronephros has begun to atrophy. The generative organs are 

 formed in a larva of about 35 mm. Shortly before the metamorphosis the 

 portion of the cloaca into which the segmental tubes open becomes separated 

 off as a distinct urinogenital sinus, the walls of which become perforated by 

 the two abdominal pores. 



The Ammocoetes of Petromyzon Planeri lives in the mud in 

 streams. Without undergoing any marked changes in structure 

 it gradually grows larger, and after three or four years undergoes 

 a metamorphosis. The full-grown larva may be as large or 

 even larger than the adult. The metamorphosis takes place 

 from August till January. The breeding season sets in during 

 the second half of April ; and shortly after depositing its 

 generative products the Lamprey dies. The changes which 

 take place in the metamorphosis are of a most striking 

 kind. 



The dome-shaped mouth of the larva is replaced (fig. 47) by 

 a more definitely suctorial mouth with 

 horny cuticular teeth (fig. 49). The 

 eyes appear on the surface ; and the 

 dorsal fin becomes more prominent, 

 and is divided into two parts. 



Besides these obvious external 

 changes very great modifications are 

 effected in almost all the organs, which 

 may be very briefly enumerated. 



1. Very profound changes take 

 place in the skeleton. An elaborate 

 system of cartilages is developed in 

 connection with the mouth ; the 

 cranium itself undergoes important 

 modifications ; and neural arches be- 

 come formed. 



2. Considerable changes are effected in the gill pouches, 

 and, according to Schneider, whose statements must however be 

 received with some caution, the branchial sack becomes detached 

 posteriorly from the oesophagus, the oesophagus then sends 

 forwards a prolongation above the branchial sack which is at 

 first solid. This prolongation forms the anterior part of the 



72 



FIG. 49. MOUTH OF PE- 

 TROMYZON MARINUS WITH ITS 

 HORNY TEETH. (From Gegen- 

 baur; after Heckel and Kner.) 



