

VEKTEBRATA 187 



of two dissimilar cells. In most aquatic forms, eggs 

 and sperm are turned loose in the water. They unite 

 and the new cell divides and differentiates until an ani- 

 mal like the parent is formed. In higher animals, or 

 rather in land animals, the process is similar, except that 

 the cells cannot be turned out in the air, where they 

 would die, but unite in the tube which leads from the 

 ovary to the outside. There they develop to a greater 

 or less degree before they pass to the exterior. 



General Characteristics. — There are no external signs 

 of segmentation in the vertebrates, but the internal parts 

 are arranged with special reference to the segmented 

 vertebral column and spinal cord. 



Except for localized hardening of the skin found, for 

 example, in the scales of fish and reptiles, or in the 

 hoofs and nails of other forms, there is no external 

 skeleton. There is, however, an internal skeleton con- 

 sisting of a skull and spinal column which supports ap- 

 pendages, sometimes unpaired but usually paired. 



The central nervous system, consisting of a brain and 

 spinal cord, is dorsal in position. The sense organs are 

 well-developed, especially the eyes and the ears. 



Gills are present in aquatic forms, lungs in terres- 

 trial forms. Gill slits are present in them all, but they 

 are lost in higher forms in adult life. 



The circulatory system is closed. The heart lies 

 ventrally in the pericardium. It is divided into an auri- 

 cle and ventricle in gill-breathing forms, and contains 

 only venous blood. When lungs are present it is fur- 

 ther subdivided into right and left halves so that arterial 

 is separated from venous blood. 



