I.] THE FROG. II 



A transverse section of the head in the region of the eyes 

 will shew, in the middle line, a dorsal cavity in which the 

 anterior part of the cerebro-spinal axis, the brain, is con- 

 tained, separated by the solid floor of the skull from a 

 ventral cavity, the mouth. 



A transverse section of the trunk will shew a dorsal 

 cavity containing the posterior part of the cerebro-spinal 

 axis, the spinal cord, separated by the solid floor of the 

 vertebral column from a ventral cavity enclosed by the 

 alimentary canal and continuous with that of the mouth. 

 But the backward continuation of the alimentary canal lies 

 within the large pleuroperitoneal chamber, of which there is 

 no indication in the head. 



If a transverse section of the trunk of the Frog be com- 

 pared with one across the middle of the body of the Crayfish 

 or Lobster (Chapter n.) it will be seen that while the chief 

 nervous centre is on one side of the alimentary canal and 

 the heart on the opposite side in both cases, the face of the 

 body on which the nervous centre lies is that on which the 

 Crayfish or Lobster naturally rests, while in the Frog it is 

 the reverse. The limbs are turned towards the neural side 

 in the Crayfish and away from it in the Frog, and the like 

 difference obtains between all Arthropoda and all Vertebrata. 



Using the term skeleton, in its broadest sense, for the 

 framework which protects, supports and connects the various 

 parts of the organism, it consists in the Frog of four kinds of 

 tissue ; the Horny, the Osseous, the Cartilaginous and the 

 Connective. Moreover, the hard parts are either developed 

 in the integument, constituting an exoskeleton, or they are 

 deeper seated and belong to an endoskeleton. 



The exoskeleton in the Frog is almost absent, being 

 represented only by the horny coating of the calcar. 



