2o COMPARATIVE ANATOMY 



doctrine of the common descent of all the animals composing a 

 phylum from an ancestor which embodied all the fundamental 

 peculiarities of structure which distinguish the phylum from 

 all other phyla. But the justification of this doctrine can only 

 be understood after a considerable study of the internal as well 

 as the external anatomy of different kinds of animals. 



Turning now to the internal anatomy of the frog, we will 

 begin by a general consideration of the principal organs and 

 systems of organs, without at first going into very exact details 

 concerning them. 



The body, we have seen, is covered by a smooth moist skin 

 or integument, which lies rather loosely so that it can easily 

 be pinched up in folds without including the underlying parts. 

 Cutting through the skin with knife or scissors we find that it 

 is attached by sheets of a white glistening substance to the 

 flesh beneath. This white substance is called connective 

 tissue, and it does not tie down the skin evenly in all places, 

 but only along certain lines, so that large spaces are left 

 between skin and flesh which are filled with a colourless fluid 

 called lymph. The most important of these spaces, known as 

 lymph-sacs, are (i) a dorsal lymph-sac, extending over nearly 

 the whole of the dorsal surface of the head and trunk, and 

 separated on either side by a septum from (2) the lateral lymph- 

 sacs, which occupy the flanks of the animal ; (3) the ventral 

 lymph-sac, a large triangular sac extending from the breast 

 region over the abdomen on the ventral surface ; (4) the 

 pectoral lymph-sac, lying over the region of the breast ; and (5) 

 the submaxillary lymph-sac, occupying the ventral surface of the 

 head and throat. There are other lymph-sacs on the fore and 

 hind limbs. These large sacs are connected with other lymph- 

 spaces lying between the muscles and organs of the body, and 

 the fluid which they contain is eventually carried into the 

 blood stream. 



After the skin has been stripped off, the body is seen to be 

 covered with muscle, commonly called the "flesh" of the 

 animal. The use and mode of contraction of the muscles has 

 already been explained (p. 6). We need only notice here 

 that the muscles which are exposed on the removal of the skin 

 belong to the system known as voluntary muscles ; by their 

 means all the voluntary movements of the body are effected. 

 Over the back, flanks, and belly the muscles have, for the most 



