nnd their Embryonic Development. 245 



tural group, as we have even among the full-grown ones, 

 what we may call transitions between the extreme forms ; 

 for instance, sharks with more elongated body than others, 

 with more extensive vertical fins, even with two dorsals and 

 some without ventrals. Again, the remarkable form of 

 skates arises solely from an extraordinary development of 

 the pectorals ; they are nevertheless closely allied to sharks, 

 notwithstanding the striking difference in the position of the 

 gill opening. 



As for the anatomical differences which exist among these 

 fishes, and upon which so much stress is placed as to make 

 the want of a heart, in Amphioxus, the foundation for a pe- 

 enliar class to include that single fish, let us not forget that 

 there is an epoch in embryonic life, when no vertebrated ani- 

 mal has yet a heart ; when the vertebral column is a mere 

 soft continuous cord ; when the brain is scarcely subdivided 

 into lobes ; when the head, as such, is not yet distinct from 

 the trunk ; when the mouth is a mere circular opening at 

 the anterior extremity of the body ; when the gills are simple 

 fissures on the sides of the head, or what is to be a head, 

 without branch iostegal rays, or operculum, or protecting 

 covering of any kind. 



Whoever is familiar with the anatomy of fishes must per- 

 ceive, after these remarks, that the peculiarities which cha- 

 racterise Petromyzon have a bearing upon the embryonic 

 condition of their structure, even in their full-grown state, 

 and do not, by any means, mark a difi'erence between them 

 4ind the sharks and skates, any more than between them 

 and any other family of fishes. On the contrary, should it 

 be possible, after these statements, to shew that there are 

 important characters, common to Petromyzon, sharks, and 

 skates, notwithstanding their extreme external differences, 

 it should be acknowledged that Cyclostomata and Plagios- 

 tomata are only different degrees of one and the same great 

 type. Now, such characters we have ; in the first place, in 

 the structure of the mouth, which differs so widely from 

 that of the other fishes, and agrees so closely in all Pla- 

 coids, as Miiller himself has shewn in his Anatomy of My- 

 xinoids. Next, the teeth also agree, in being arranged in 



