158 ESSENTIALS OF ZOOLOGY 



and reach a large size in some species, as the thornback and 

 starry ray. Their position and the rows which they form are 

 important features in determining species. The scales may 

 also occur on the ventral surface. 



INTERNAL STRUCTURE. Bony formation is thus evidenced 

 in the development and structure of the placoid scales. The 

 skeleton, however, is a cartilaginous one, and it remains car- 

 tilaginous, only strengthened by superficial calcifications in the 

 skull and in the centra of the vertebrae. Cartilage is a con- 

 nective tissue in which a matrix is formed between the cells 

 and changed chemically with the formation of the compound 



Dorsal roots Neural canal 



^ Notochord 

 Haemal process Centrum 



FIG. 77. Raia. A portion of a sagittal section of an embryo skate, illustrat- 

 ing the relationship of the arches of the vertebral column, and a more 

 magnified part of one of the arches to illustrate its histological structure. 



called chondrin. The cells have the power of multiplication, 

 and thus the cartilage may be enlarged as required. It forms 

 a tough, resistent, elastic tissue, and it is not penetrated by 

 blood-vessels nor by nerves (fig. 77). It is developed primarily 

 in the sheaths around the notochord and nerve cord and in the 

 head region. 



Skeleton. The skeleton is resolved into axial and appendi- 

 cular. The axial consists of the vertebral column, ribs, and 

 skull ; the appendicular, of the skeleton of the limbs and their 

 girdles, and of the median fins and their supports. 



The vertebral column is developed in the products of the 

 sclerotomes which ensheath the notochord and the nerve cord, 

 the notochord being surrounded by an inner perichordal 

 sheath and an outer skeletogenous sheath. In many fishes 

 the chondrification takes place in a series of neural and haemal 

 processes, confined in the case of the notochord to the skeleto- 



