114 



SUB-CLASS MARSIPOBRANCHII (CYCLOSTOMATA). 



coetes is only partly developed and lies be- 

 neath the skin. In the lamprey it becomes 

 fully developed and travels to the surface. The 

 pericardium of Ammocoetes opens into the 

 general body cavity, but the two become 

 completely separate in the adult. The an- 

 terior part of the kidney which had been 

 developed in the Ammocoetes disappears, and 

 a fresh posterior part is formed. The prone - 

 phros had already begun to disappear dur- 

 ing the Ammocoetes stage, and the portion 

 of the cloaca into which the urinary ducts 

 open becomes separated off as a urogenital 

 sinus shortly before the metamorphosis. 



The skeleton undergoes very considerable 

 change at the metamorphosis. The Ammo- 

 coetes is without the cartilaginous dorsalia or 

 neural arches in the trunk region. These 

 appear at the metamorphosis as do cartilages 

 of the mouth, and the side walls and roof of 

 the skull. The spinal cord, which is nearly 

 round in section in Ammocoetes, becomes 

 flattened at the metamorphosis. 



The head muscles of Ammocoetes are entirely 

 destroyed and reformed at 

 the metamorphosis. 



The thyroid body arises in 

 the embryo as a groove in 

 the branchial region of the 

 g pharynx. The opening soon 

 becomes narrowed to a pore 

 placed between the second 

 and third permanent* branch- 

 ial pouches (Fig. 60). In Am- 

 mocoetes the tube so formed 

 becomes divided and assumes 



FlQ/59. A. Ammo- 

 coetes of Petromy- 

 zon planeri, 2 ins. 

 long, side view 

 (after W. K. Park- 

 ,er) brl first, br7 

 seventh branchial B Ventral view of 



* There is said to be a trace of an 

 eighth pouch, in front of the first 

 permanent one, in the embryo and young larva. It is supposed to repre- 

 sent the hyomandibular cleft of other fishes, but it never acquires gill folds 

 or an external opening. 



aperture ; na na- 

 sal aperture; eye. 



the head of the 

 same larva. 



