THE RABBIT. 5 



of air-breathing vertebrata, there are either five or less than 

 five digits on both fore and hind limbs : if there are fewer 

 than five it is always possible to determine, by comparison 

 with other animals, which of the full set have disappeared. 



4. The Hair. Practically the whole surface of the 

 body is covered by hair. This covering is distinctive of 

 that class of the vertebrata called mammals, which in- 

 cludes man, most of the larger and commoner land animals, 

 and the whales and manatee. In birds we have an equally 

 characteristic cover in the feathers, the frog is naked, and 

 among the fishes we find either naked or scaly skins. 



5. Internal Structure. In the actual examination of 

 the rabbit's internal structure, the student will proceed by 

 the method of dissection, or the separation of the organs 

 from one another. To obtain a clear notion of the natural 

 position and mutual relations of the parts, another method 

 is useful. This is the method of taking sections across various 

 regions of the body. Diagrammatic representations of such 

 sections are given in fig. 2. 



6. Section of Abdomen. Taking first the section 

 through the abdomen, we notice that two regions are seen 

 a smaller dorsal region, occupied by the backbone (with its 

 contained spinal cord) and a considerable mass of muscle ; 

 and a ventral region, in which there is a large cavity the 

 body-cavity or ccelom, enclosed by a comparatively thin mass 

 of muscle (the body-wall) and containing a greatly coiled 

 tube, the alimentary canal or gut. The ccelom, here and 

 elsewhere, contains a watery fluid (serous fluid). The body- 

 wall is lined by a membrane called the peritoneum. If 

 this is traced round from either side to the dorsal region 

 near the middle line, it is found to be reflected ventral- 

 wards, as a double membrane the mesentery which sur- 

 rounds, and hence suspends, each of the coils of the gut. 

 A little on either side of the middle line the kidneys bulge 

 ventralwards into the ccelom, pushing the peritoneum before 

 them. Evidently, however, the relation of the kidneys to 

 the ccelom differs from that of the gut only in degree, 



