344 THE PINEAL ORGAN 



sources, Stensio has classed vertebrate Crania into two main branches, 

 namely, those without jaws — Agnathi ; and those with jaws — Gnatho- 

 stomi. The Agnathi comprise the Class Ostracodermi or Cyclostomata, 

 which he subdivides into two subclasses with their respective Orders as 

 shown in the following table : 



VERTEBRATA CRANIATA 



Branch I : Agnathi 



Class : Ostracodermi (Cyclostomata) 

 Subclass A : Pteraspidomorphi 

 Order i : Heterostraci 

 „ 2 : Palaeospondyloidea 

 „ 3 : Myxinoidea 

 Subclass B : Cephalaspidomorphi 

 Order I : Osteostraci 

 „ 2 : Anaspida 

 „ 3 : Petromyzontia 



Branch II : Gnathostomi 



Stensio concludes that the Ostracodermi constitute a group of primarily 

 agnathous craniate vertebrates " which have nothing to do with either the 

 Arthrodira or the Antiarchi, and are true fishes related to the Elasmo- 

 branchs." 



Of the two living Orders of the Ostracodermi, the Myxinoidea (hag- 

 fish) is included in the pteraspid Subclass A and the Petromyzontia 

 (lampreys) belong to the cephalaspid Subclass B. 



In the subclass Pteraspidomorphi the rostral part of the head formed 

 by the ethmoidal region and the common naso-hypophyseal opening lie 

 on the ventral side of the head, close in front of the mouth, whereas in 

 the subclass Cephalaspidomorphi the rostral part of the head is formed 

 by the excessively developed upper lip, and is thus of visceral origin ; 

 moreover, owing to the great development of the upper lip the naso- 

 hypophysial opening is carried upward on to the dorsal aspect of the 

 head, where it lies immediately in front of the pineal foramen. Stensio 

 also concludes that the hag-fishes and lampreys being persistent repre- 

 sentatives of the extinct Ostracoderms must be a primitively low type of 

 agnathous craniate vertebrate and not degenerate descendants of the true 

 fishes, but it is also quite clear with regard to the skeleton that they have 

 undergone regressive changes, and that the absence of pectoral fins, 

 which is characteristic of the living species, is one of these secondary 

 regressive changes. 



We may add, further, that the degenerate condition of the parietal 

 sense-organs, more especially of the left pineal eye, is an additional fact 



