GENERAL ZOOLOGY AND COMPARATIVE ANATOMY 19 



The circulation is a mere water vascular sys- 

 tem in the lowest aquatic forms, in which there 

 is no corpusculated or true blood, and no circula- 

 tory organs. In the higher Invertebrates there 

 is true blood, colorless or greenish, with true veins 

 and arteries. In the insects there is a distinct 

 heart with one ventricle. In the mollusca there is 

 a heart with one valve which propels the blood 

 both ways alternately. Some forms have a biloc- 

 ular heart. 



Eespiration is performed in the lowest forms 

 by tentacles or cilia, and the higher aquatic forms 

 have cilia or gills. In the insects the blood is 

 aerated by circulation of the air in the pulmonary 

 tubes which ramify throughout the body. The 

 snails breathe by means of an air-sac with a 

 ciliated lining. 



Locomotion is performed by various means : by 

 tentacles and cilia in the lower forms; by legs 

 and wings in the insects ; by legs in the Crustacea ; 

 by a fleshy peduncle in the mollusca, etc. 



Eeproduction is performed by fission, budding, 

 etc., in the lowest forms; the laying of gelatinous 

 eggs in the higher orders, etc. Some forms, as 

 the insects, undergo a series of metamorphoses 

 before attaining the mature stage. 



The sub-kingdom of the Vertebrates comprises 

 all animals which have an internal backbone or 

 vertebral column composed of articulated verte- 



