30 THE FROG chap. 



axial poiiimu the head and trunk, and of two pairs of lateral 

 offshoots or appendages, the fore- and hind-limbs. The trunk 

 is hollowed out into two cavities : the abdominal or body- 

 cavitv {iKloinc) below, and the neural canal above ; of these 

 the neural cavity alone is continued into the head. The 

 abdominal cavity contains the greater part of the enteric canal, 

 the liver, gall-bladder, pancreas, spleen, lungs, heart, kidneys, 

 urinary bladder, and reproductive organs. The neural 

 canal contains the brain and spinal cord. The anterior 

 end of the enteric canal is continued forwards into the head, 

 forming the mouth-cavity, and opens externally by the 

 mouth aperture ; its posterior end opens externally by the 

 anus. The enteric canal passes through the containing 

 body-cavitv, having no communication with it. The lungs 

 open into the pharynx, and thus communicate with the ex- 

 terior not only by the mouth but also by the nostrils. The 

 kidneys, bladder, and oviducts communicate with the 

 cloaca, and thus with the exterior through the anus. Neither 

 the neural nor the abdominal cavity has any communi- 

 cation with the exterior. The walls of the head and trunk 

 consist largely of muscles and bones covered with skin. 

 The limbs are solid outgrowths of the trunk, formed also 

 mainly of muscle, with bony supports and a covering of 

 skin. 



Organs. — Notice that the body consists of various definite 

 structures, or organs as they are technically termed, which 

 ha\'e various purposes ox functions to perform. The enteric 

 canal, together with the liver and pancreas, are organs of 

 diges/ion ; the lungs and skin, organs of respiration orhxea.ih- 

 ing ; the heart and blood-vessels organs of circulation, serving 

 as they do to propel and conduct the blood through the body ; 

 the kidneys, aided by the skin, organs of e.xortion, for getting 

 rid of waste matters ; the ovaries and spermaries, organs of 



