44 ANATOMY OF THE RABBIT 



7. The formation of the circulatory system primarily on an 

 aquatic plan. This involves the formation of (a) the heart in a 

 position ventral to the digestive tube and immediately behind the 

 gills; (b) a ventral aorta, passing forward to the gills, and dividing 

 into a paired series of branchial aortic arches; (c) a dorsal aorta, 

 in which the upper ends of the aortic arches unite, and which 

 passes backward along the ventral surface of the axial support; 

 and (d) a series of paired veins returning the blood from various 

 parts of the body to the heart. 



8. The formation of the reproductive organs or gonads in 

 association with the dorsal lining of the coelomic cavity, and their 

 connection with the outside of the body by modified kidney ducts. 



9. The formation of the kidneys, either as embryonic or as 

 permanent structures, from an intermediate mass of tissue, lying 

 in general between the dorsal musculature and the lining of the 

 coelomic cavity (cf. position of embryonic kidney in Fig. 22). 



10. The formation, in the ventral portion of the body, of an 

 extensive space, the coelomic cavity or coelom, afterwards differ- 

 entiated into pericardial, pleural, and peritoneal portions. 



The Skeletal System 



The designation "vertebrate" has reference to a common feature 

 of fishes, amphibians, reptiles, birds, and mammals — the possession 

 of a backbone or vertebral column, composed of individual seg- 

 ments, the vertebrae. Vertebrates are, however, more properly 

 described as animals having an internal skeleton. 



Skeletal Architecture 



The skeleton being composed of nearly rigid materials, it is 

 necessary, in order that movement may be possible, that these 

 should form many separate pieces, designated cartilages or bones 

 according to the material composing them. In the embryonic 

 condition, cartilage rudiments form a complete but primitive 

 skeleton and in some lower vertebrates the skeleton remains entire- 

 ly cartilaginous throughout life. The latter, however, is probably 

 a degenerate condition for, although cartilage may actually have 

 originated earlier than bone in the history of living beings, bone 



