Ixxx Introduction. 



smaller, the segmentation of the yolk is more extensive than in 

 Elasiiwbranchii, but less so than in Mammals or Amphibia. 



In Marsijjohranchii, as in Amphibia and Ampldoxus, the intes- 

 tinal canal is formed within the yolk sac, as the result of an 

 invagination commencing at the future anus, proceeding from 

 without inwards, and forming thus a tube without any rmibilicus, 

 within a cavity which is at once yolk cavity and peritoneal sac. 



Fishes, like Amphibia, are competent not rarely to sexual func- 

 tions before they are mature in other particulars. Their power 

 of repairing injuries, and reproducing lost parts, is confined to the 

 fins. The capacity for growing as long as life lasts, which some 

 Fishes are said to possess, may be exj)lained by the facts that their 

 bodies are, firstly, of very nearly the same specific gravity as the 

 w^ater in which they live ; and, secondly, of a temperature which is 

 but a very little higher than that which they are there exposed 

 to. Thus the force which in other animals is expended in the 

 way of opposition to that of gravity, and in the way of producing 

 heat, is available for sustaining continuous growth. 



The Class Pisces is divided into six orders — the D'qmoi, the Elas- 

 mobranchii, the Ganoidei, the Teleostei, the Marsipobranchii, and 

 the Pharyngohranchii. Of these the Dipnoi are to be considered 

 the highest, as presenting in their organization many points of 

 affinity to the Amphibia; the Elamiohranchii and the Ganoidei 

 have the oldest known geological history of any members of the 

 class ; they possess many characters in common with each other 

 and with the Dipnoi^ such as the abdominal position of the posterior 

 pair of limbs ; the retention ordinarily of more or less of the axial 

 endoskeleton in a cartilaginous condition ; the possession of an 

 anterior functional uniserial gill, which is lost as such in osseous 

 Fishes; and the possession of a spiral intestinal valve. Though 

 coeval in geological time with the Ganoidei, the Elasmobranchii are 

 a distinctly specialized, whilst the Ganoidei are a generalized type 

 of Vertebi-ata. The Teleostei, and amongst them the Physostomi 

 especially, are linked by many affinities to the Ganoidei. The Mar- 

 sipobranchii may be looked upon as representing a very low grade of 

 development of the type upon which the Elasmobranchii are con- 

 structed ; the superaddition of specializations has however proceeded 

 so far as to leave few points of positive similarity beyond those which 

 an endoskeleton in great part or wholly cartilaginous ; a branchial 



