XIII PHYLUM CHORDATA (ill 



In embryology, eggs with much food-yolk are to be looked upon 

 as more modified than those with little, unless there is distinct 

 evidence of reversion towards an alecithal condition. Any special 

 contrivances for the nourishment and protection of the embryo, 

 obviating the necessity for the production of immense numbers of 

 eggs, are also marks of advance. 



On both these lines of evidence the lowest place may safely 

 be assigned to the Cyclostomes. In spite of the similarity of 

 the lateral cartilage of the Lamprey to Meckel's cartilage, 

 there is no real evidence that the ancestors of the class had 

 either jaws or limbs, and the most reasonable theory is that they 

 are the descendants — highly specialised in certain respects in 

 accordance with their peculiar mode of life — of a primitive 

 craniate stock. 



With regard to the two largest groups of Pisces — the Elasmo- 

 branchii and the Teleostei — the evidence from anatomy and 

 embryology is conflicting. The Teleostei take the highest place 

 in virtue of their skeleton, operculum, air-bladder, and gills, as 

 well as in their extraordinary adaptability to various environments ; 

 but the Elasmobranchs reach a distinctly higher grade of organi- 

 sation in their enteric canal, heart, brain, and urinogenital organs, 

 as well as in their large and well-protected eggs. The anatomy 

 of Ganoids seems to show, however, that the spiral valve, conus 

 arteriosus, and typical oviducts (Mtillerian ducts) have been lost 

 in the course of the evolution of the Teleostei, and that the simpler 

 structure of these organs in that order is actually a concomitant 

 of their extreme specialisation. 



The Holocephali and the Dipnoi, while agreeing with Elasmo- 

 branchs in many important respects, show an advance in the 

 presence of an autostylic skull and of an operculum, while the 

 Dipnoi rise above all other Fishes in possessing not only lungs 

 like Polypterus, but also posterior nares and a partially divided 

 auricle. The lung appears to have been derived from an air- 

 bladder with pneumatic duct opening on the ventral wall of the 

 pharynx, as in Polypterus ; by the dorsal shifting of the duct 

 and its final atrophy the closed air-bladder of the higher Teleostei 

 has arisen. 



Coming to the results of Paleontology, many striking and 

 unexpected facts have recently come to light. There is reason 

 to believe that PaliEospondylus is a Cyclostome, but one with well- 

 developed vertebrae ; from which it must be assumed either that 

 the vertebral column of existing members of the class is degenerate, 

 or that Palaeospondylus is a highly specialised offshoot of the 

 primitive Cyclostome stock, in which a vertebral column had been 

 independently acquired. The latter conchision seems the more 

 probable, and is supported by the fact that in all three orders of 



