x CYCLOSTOMATA 401 



are comparatively undeveloped, the semicircular canals In in^ reduced 

 to two in the Lampreys and one in the Myxinoids. 



Making a general survey of their features we see that these Cyclostomes 

 are a remarkably interesting group of vertebrates. They present an 

 interesting medley of characters some primitive, some specialized, some 

 doubtful. Among the first we may group the negative features that 

 the notochord never becomes replaced by a segmented vertebral column, 

 that the cranium does not extend back beyond the origins of cranial 

 nerves IX and X, that the pituitary ingrowth never becomes separate 

 from the outer skin, that in the Myxinoids the pericardiac portion of the 

 coelome does not separate from the peritoneal. Of positive features 

 there is the occurrence of a velum, and in the young Lamprey of an 

 endostyle as in Amphioxus. 



As specialized features we may interpret the saccular gills, the 

 modifications of the branchial skeleton, the simplification of the kidney 

 in the Myxinoids. 



Very doubtful features are the absence of scales and of paired limbs. 

 It is natural to regard their absence as primitive, to regard the Cyclostomes 

 as persisting survivors of the ancient vertebrates which had not yet 

 evolved scales or limbs. But we have seen that in other groups (e.g. 

 Siluridae) a scaly covering may disappear without a trace : we shall see 

 similarly that in vertebrates with elongated bodies it is quite a character- 

 istic feature that their limbs tend to become reduced and eventually to 

 disappear entirely. And there is no convincing evidence that this has 

 not happened in the Cyclostomes. 



BOOKS FOR FURTHER STUDY 



The Cambridge Natural History, Vol. VII. 

 * Starr Jordan. A Guide to the Study of Fishes. 

 Goodrich, Fishes, in Lankester's Treatise on Zoology, Part IX. 



2 D 



