XIII.] THE i'KOG. 161 



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

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

 separated by the solid floor of the skull from a ventral cavity, 

 the mouth. 



A transverse section of the abdomen will shew a dorsal 

 cavity containing the posterior part of the cerebro-spinal 

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

 tebral column from a ventral cavity containing the ali- 

 mentary canal and continuous with that of the mouth. 

 But the backward continuation of the alimentary canal is 

 embraced by the large pleuroperitoneal chamber, of which 

 there is no indication in the head. 



On comparing the transverse section of the abdomen of 

 the Frog with a transverse section of the middle of the body 

 of the Lobster, 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, there is no solid and com- 

 plete partition between the nervous centre of the Lobster 

 and the alimentary canal. Moreover, the face of the body 

 on which the nervous centre lies, is that on which the Lobster 

 naturally rests, while in the Frog it is the reverse. The limbs 

 are turned towards the neural side in the Lobster and away 

 from it in the Frog, and the like difference obtains between 

 all Vertebrata and all Arthropoda. 



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 Con- 

 nective. Moreover, the hard parts are either developed in the 

 integument, constituting an exoskeleton, or they are deeper 

 seated and belong to the endoskeleton. 



Leaving aside a question that may arise as to the nature 

 of some of the cranial bones, the exoskeleton in the Frog is 

 M. 11 



