,570 ZOOLOGY SECT. 



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

 pair of low ridges at the sides of the anus to vestigial lateral fins, 

 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, cerebral hemispheres, and typical oviducts (Miillerian 

 .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 

 Pipnoi rise above all other Fishes in possessing lungs, 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 Palaeontology, many striking and 

 unexpected facts have recently come to light. There is reason 

 to believe that Palseospondylus 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 Pala3ospondylus is a highly specialised offshoot of the 

 primitive Cyclostome stock, in which a vertebral column had been 

 independently acquired. The latter conclusion seems the more 

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

 Ganoids there are some species with a persistent notochord, others 

 with well-ossified vertebras, the conclusion being that the vertebral 

 column is a polyphyletic structure, that is, has been evolved 

 independently in various groups in accordance with similar 

 conditions. 



Among extinct Elasmobranchs the Acanthodea and Pleuracan- 

 thea had membrane bones investing the cranium, and Cladoselache 

 had no claspers. These facts seem to indicate as a probable an- 

 cestor of the Teleostomi and Dipnoi- the two sub-classes with 



