XIII 



PHYLUM CHORDATA 



133 



The brain, in spite of its small size, is in some respects notably 



iri the presence of cerebral hemispheres of a more advanced 



type than that of some of the true Fishes. The circumstance 



that the pituitary pouch perforates the 



skull-floor from above and becomes early 



associated with the olfactory sac, is 



unique among the Vertebrata. The 



kidney of Bdellostoma is of the most 



primitive type, and the presence of a 



large pronephros is a significant archaic 



character. The total absence of limbs 



may be a result of degeneration. 



The geographical distribution of the class 

 is interesting from the fact that each order 

 contains some genera which are mainly 

 northern, others which are exclusively 

 southern. Petromyzon is found on the 

 coasts and in the rivers of Europe, North 

 America, Japan, and West Africa : it is 

 therefore mainly Holarctic. Ichthyomyzon 

 is found on the western coasts of North 

 America, Mordacia in Tasmania and Chili, 

 Geotria in the rivers of Chili, Australia, 

 and New Zealand. Myxine occurs in the 

 North Atlantic and on the Pacific Coast 

 of South America ; Bdellostoma on the 

 coasts of South Africa, New Zealand, 

 and Chili. 



Until quite recently no undoubted 

 fossil remains of Cyclostomes have been 

 known, but there is some reason to believe 

 that a little fossil fish, Palccospondylus 

 gunni (Fig. 765), lately discovered in the 

 Devonian rocks of Scotland, is referable 



-i: 



^v 



to this class. It is about an inch long 

 and shows two regions, the cranium and 

 the vertebral column ; there is no trace 

 of jaws, branchial basket, or limbs. The 

 vertebral column is composed of calcified 

 centra with neural arches : haemal arches 

 are present in the caudal region ; the 

 structure of this part of the skeleton is 

 thus of a distinctly higher type than in re- 

 cent Cyclostomes, and lends support to the view that the latter are 

 degenerate. There is a caudal fin supported by forked rays. The 

 cranium consists of an anterior, probably trabecular, region (t.p.\ 

 and of a posterior region (p. .) which seems to answer to the 



Km. 765. Palseospondylus 

 gunni (magnified), c. cirri ; 

 />.. parachordal and auditory 

 region ; t. p. trabecular re- 

 gion ; x. backward processes 

 of skull. (After Traquair.) 



