54 



EVOLUTION AND GENETICS 



relatively simple. The head contains no bony shell encasing the 

 brain but does include a mass of cartilage of peculiar form in which 

 the brain rests. This structure is called the chondrocranium (Fig. 

 35). The vertebral column consists of a series of vertebrae with 

 hollow ends, hour-glass-shaped in longitudinal section, between 



Eeproductlve ,\V 

 ceU 



— Stlgmar 



'» Contractile 

 vacuole 



Hacrogametes - 



Microgametes 



Fig. 32. — Volvox globator, a colonial unicellular organism. A, a sexually ripe 

 colony showing reproductive cells in various stages; B, a portion of the edge 

 of the colony highly magnified. (From Hegner, after Bourne and Kolliker.) 



which small remnants of the notochord persist. Associated with 

 the head and pharyngeal region is the visceral skeleton, consisting 

 of the upper jaw (pterygo-quadrate cartilage), lower jaw (Meckel's 

 cartilage) and six pairs of cartilages behind the mouth, the first 

 constituting the hyoid arch and the remainder supporting the 

 gills and called branchial arches. The appendicular skeleton con- 

 sists of two horseshoe-shaped pieces of cartilage in the pectoral 

 and pelvic regions respectively, each bearing a pair of fins based on 

 cartilage supports. 



