THE FROG AND VERTEBRATES IN GENERAL 153 



(2) Vertebrate animals possess an internal supporting skeleton 

 (endoskeleton) composed essentially of living matter. The 

 bones, tendons, cartilage and the very abundant connective tissues 

 of all kinds constitute the endoskeleton. In addition to the ver- 

 tebral column there are : (a) a brain case, or cranium, in which 

 special locations, the sense-capsules, are present for the olfactory, 

 optic, and auditory sense organs ; and (6) supports for the append- 

 ages. Vertebrates have either a partial or complete exoskeleton 

 composed essentially of non-living matter. Examples of the exo- 

 skeleton are to be seen in the scales of Fishes, the shell of a Turtle, 

 the feathers of Birds, the hair, nails, hoofs, and claws of Mammals. 

 Exoskeletal structures have previously been noted in our study of 

 certain Invertebrates, but the bony endoskeleton of the Vertebrate 

 is a new feature. 



(3) The vertebrate limbs are restricted in number. There are 

 never more than two pairs present, and in many cases there are 

 less. Thus in certain Reptiles like the Snake, limbs are lacking. 

 It appears that the five-fingered, or pentad actyl limb is to be 

 considered as typical of the Vertebrates. The entire series of 

 vertebrate limbs is regarded as homologous, and these range from 

 the many-rayed fish fin to the limbs of certain Ungulates, like the 

 Horse, which, as will be seen later, have retained only one func- 

 tional digit on each limb. 



(4) The blood of the Vertebrates contains a new type of blood 

 cell, the red corpuscle. The color of these cells is due to the im- 

 portant respiratory compound, hemoglobin, which, in the Earth- 

 worm, is carried in the blood plasma, rather than in specific cells. 



(5) Reproduction is always sexual. There is also an absence of 

 hermaphroditism in the Vertebrates. The abandonment of both 

 asexual reproduction and hermaphroditism appears to have been 

 a comparatively recent step in the history of animal development. 

 As a matter of fact, examples of both these discarded methods of 

 reproduction are to be found in the lowest Chordates, as noted 

 above. 



In addition to the distinctive vertebrate characters, as just 

 noted, attention should be called to the five important structural 

 features, first noted in the discussion of the Earthworm, which 

 are, in general, characteristic of all the higher Metazoa and which 

 are well-exhibited by the vertebrate organism. These features it 

 will be remembered are (a) the triploblastic condition with ecto- 



