CYCLOSTOMATA. 



which nearly, though not quite, correspond in number with the bran- 

 chial pouches. 



The growth of the myotomes would seem, as might be anticipated from ( 

 their independent innervation, not to be 

 related to that of the branchial pouches, 

 so that there is a want of correspondence 

 between these parts, the extent of which 

 varies at diffei'ent periods of life. The re- 

 lation between the two in an old larva is 

 shewn in fig. 47. 



The head of the larva of Petromyzon 

 differs very strikingly in general appear- 

 ance from that of the normal Vertebrata. 

 This is at once shewn by a comparison 

 of fig. 43 with fig. 29. The most im- 

 portant difference between the two is 

 due to the absence of a pronounced 

 cranial flexure in Petromyzon ; an ab- 

 sence which is in its turn probably 

 caused by the small development of the 

 fore-brain. 



The stomodaium of Petromyzon is 

 surprisingly large, and its size and 

 structure in this type militate against 

 the view of some embryologists that 

 the stomodseum originated from the 

 coalescence of a pair of branchial pouches. 



In the region of the trunk there is present an uninterrupted 

 dorsal fin continuous with a ventral fin round the end of the tail. 



There is a well-developed body cavity, which is especially dilated 

 in front, in the part which afterwards becomes the pericardium. In 

 this region is plajced the nearly straight heart, divided into an auricle 

 and ventricle (figs. 42 and 43), the latter 

 continued forwards into a bulbus arteri- 

 osus. 



The myotomes are now very numerous 

 (about 57, including those of the head, in 

 a three days' larva). They are separated 

 by septa, but do not fill up the whole space 

 between the septa, and have a peculiar 

 wavy outline. The notochord is provided 

 with a distinct sheath, and below it is 

 placed a subnotochordal rod. 



The alimentary canal consists of a 

 narrow anterior section free from yolk, 

 and a posterior region, the walls of which 

 are largely swollen with yolk. The an- 



FiG. 45. Horizontal section 



THEOUGH THE HEAD OF A JUST 

 HATCHED LARVA OF PeTROMYZON 

 SHEWING THE DEVELOPMENT OF THE 

 LENS OF THE EYE. 



th.c. thalamencephalon ; op.v. 

 optic vesicle; I. lens of eye; h.c. 

 head cavity. 



Fig. 46. Eye of a larva of 

 Petromyzon nine days after 

 hatching. 



I. lens; r. retina. 



The section passes tlirough 

 one side of the lens. 



