xlii Introduction. 



cartilaginous, and may be connected by suture, and not by movable 

 articulation, with the first vertebra of the trunk. The apex of 

 the scapular arch, so far as it is constituted by true endo-skeletal 

 elements, corresponds at its first appearance to the interspace be- 

 tween the second and third vertebrae, marking off thus two cervical 

 vertebrae; but a cervical region is not by any means invariably 

 recognisable in the adult condition of these animals ; when there 

 are two occipital condyles, they are constituted by the ex-occipitals 

 alone. They never possess a series of costal arches completed in- 

 feriorly by sternal bones ; and it is only rarely that (in Amphibia) 

 there is any sternum present at all. They always possess two 

 aortic arches at least, and nearly invariably an aortic bulb. They 

 are in many cases competent to the maturation of sexual products, 

 before they attain their full size ; and in the case of the Amphibian 

 Axolotl, before the gills characteristic of the larval or tadpole-stage 

 are discarded. 



No Vertebrata are social, nor are any fixed to one spot. The 

 power of repairing injuries and mutilations is, with possibly a few 

 exceptions, confined to the cold-blooded Amphibia and Reptilia. As 

 in the two higher Sub-kingdoms of the Invertebrata, the Mollusca 

 and the Arthropoda, there are both air-breathing and water-breathing 

 representatives of this Sub-kingdom. 



As in the Sub-kingdom Mollusca, so in that of Vertebrata, there 

 are very few animals of parasitic habit. All parasitic Vertebrata 

 belong to the class Pisces, and amongst these we may mention the 

 Myxinoids, which are not only ecto-parasitic, but penetrate even 

 into the abdominal cavity of other Fishes, such as the Sturgeon. 

 A Siluroid fish has been found to inhabit the branchial cavity of 

 another fish {Flatystomus) of the same family ; and the invertebrate 

 Astenas discoidea is infested by Oxijheles lumhricoides , and certain 

 Holothurians by a Fierasfer. These latter cases, however, are 

 considered by Van Beneden to be instances of ' commensalism' 

 rather than of parasitism strictly so called. See Bulletins de 

 I'Academie Royale de Belgique, 2™® serie, tom, xxviii., no. 12, 

 pp. 624-626, 642, 643, ihique citata. 



Class, Mammalia. 

 Air-breathing, warm-blooded Vertebrata, in which the epidermis 

 developes hairs over a greater or lesser extent of the surface of the 



