332 ZOOLOGY 



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 accompanying diagrams (Fig. i6i) 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 

 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 



Fig. i6o. 



Fig. i6o. Diagram of a trunk vertebra in a Mammal, c, centrum; cA., position originally 

 occupied by the notochord; A., head of the rib; A.c, hsmal cavity; n.a., neural arch; n.c, neural 

 canal; n,s., neural spine; r., rib; St., sternum; s.o., sternal cartilage uniting ribs and sternum; t.p., 

 transverse process of vertebra; («., tubercle of rib. 



Questions on the figure. — Compare all the parts here with corresponding ones 

 of Fig. 159: A, B, and note the differences. What is gained by the articulation 

 of ribs with a sternum? What is lost? In which groups of Vertebrates is a 

 sternum found? In which are fully developed ribs found? 



radiating fin-rays covered by a membrane (Figs. 177, 178). 

 Yet it is believed that from some such primitive condition the 

 more specialized appendages have arisen. 



351. 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 (stomo- 

 daeum), 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 



