33 ZOOLOGY. 



Questions on the figure. Compare the two appendages throughout 

 and note the corresponding bones. How much is girdle? How much 

 appendage proper? How many carpals? Tarsals? Which are proximal? 

 Which distal? How do the phalanges differ? Which is the thumb? How 

 can you be sure? Compare this figure with figures (in reference texts) 

 of the appendages, botn front and rear, of the frog; of some bird; of 

 some Carnivore ; of the horse ; of man. Where are the greatest varia- 

 tions, i. c., which bones depart most from this typical condition? 



strength at the expense of freedom of motion. The first joint 

 of each appendage consists of one bone (arm or thigh) ; the 

 second, of two (forearm or shank) ; then follows a region of 

 several small bones (wrist or ankle), succeeded by the hand 

 or foot with five (usually) bones, and then by five digits 

 (fingers, toes) of a varying number of joints. The accom- 

 panying diagrams (Fig. 159) will make clear these relations, as 

 well as the names of the bones. Bones may disappear or fuse 

 with others in such a way as to cause a wide variation from 

 this type; indeed the type is perhaps never realized in any 

 single animal. In fishes the appendage and the girdle are 

 often very simple, the limb being little more than radiating 

 fin-rays covered by a membrane (Figs. 174, 175). Yet it is 

 believed that from some such primitive condition the more 

 specialized appendages have arisen. 



343. The Digestive Organs. As in many of the inverte- 

 brates which we have studied, the alimentary canal in the 

 vertebrates possesses an anterior, ectodermal portion (stomod- 

 seum), a mid-gut lined with entoderm (mesenteron), and a 

 posterior ectodermal part (proctodseum). The tract is lined 

 throughout with a mucous membrane. Outside of this are the 

 layers of unstriped muscle fjbres, circular and longitudinal, 

 by which the food is forced onward. The muscles are especi- 

 ally developed in certain regions, as in the stomach. Outside 

 of all these, in the portion passing through the body cavity, is 

 the serous membrane derived from the mesoderm, a portion 

 of the lining of the body cavity. The mucous surface which 

 is, naturally enough, the important portion in digestion and 

 absorption may be increased by the lengthening of the tube as 



