ARTHROPODA 175 



in great numbers open into the posterior part of the ali- 

 mentary canal. 



Circulatory System. — The circulatory system is more 

 highly developed in some forms than in others, but it is 

 never entirely closed. A heart lies dorsal to the ali- 

 mentary canal. The blood passes from it through the 

 arteries into great open spaces and from these it is 

 sucked back into the heart through large openings. 

 There is an intimate connection between the develop- 

 ment of the respiratory and circulatory systems. The 

 more localized the respiration, the more complete the 

 circulation ; the more it is diffused through the body the 

 more closely oxygen is brought in contact with the tissues 

 and the more the circulatory system is reduced. 



Sense Organs. — Most of these animals are sen- 

 sitive to sound vibrations and seem able to distinguish 

 substances through taste or smell; but touch and sight 

 are the most highly developed of the senses. The eyes 

 are of two kinds, simple and compound. The compound 

 eye is a collection of simple eyes closely united. Each 

 simple eye sees part of the object. It is a tube with 

 a sensitive surface at its base. The light from a single 

 point is reflected through the tube and forms an image 

 on the surface. The sum of the partial images makes 

 the complete image. 



Reproduction. — Reproduction takes place sexually. 

 Sometimes individuals are hermaphroditic, in which 

 case eggs and sperm are formed in the same animal ; but 

 usually the sexes are separate and to be distinguished 

 by a difference in such external characters as color, size 

 and form of appendages. The sexual organs are well- 



