220 SYSTEMATIC ZOOLOGY. 



always present, pouring digestive juices into the alimentary 

 canal behind the stomach. The eyes are either simple or 

 compound. In the simple eyes there is a single lens for 

 the whole structure, while the compound eyes are com- 

 posed of many separate eyes, each with its own lens. In 

 some cases the compound eyes are placed on jointed 

 stalks, at others they are on the walls of the head. Ears 

 have been found in some forms. Usually they are sacs 

 in the base of the antennulse, but in the opossum-shrimps 

 they occur near the end of the abdomen. The hairs which 

 occur over the body are organs of touch, and probably 

 some of them around the mouth and on the antennae 

 serve as organs of taste and smell as well. 



A heart is lacking in a few forms. When present it is 

 dorsal in position, but may be either in thorax or abdo- 

 men. It may be a long tube with several chambers, or 

 a short thick muscular organ without divisions. The 

 blood returning from the gills enters the heart and is 

 forced thence to all parts of the body, a condition quite 

 different from what is found in the fish. It does not 

 flow throughout its course in closed vessels, but escapes 

 from them and comes into large spaces (lacunae) between 

 the various organs and muscles, and from the largest of 

 these lacunae, near the floor of the body, it again goes to 

 the gills. 



In the Crustacea the excretory organs (nephridia) open 

 to the exterior entirely independently of the alimentary 

 canal. In the higher Crustacea (crayfish, etc.) these 

 nephridia are known as 'green glands' and open at the 

 base of the antennae (second segment of the body); in 

 the lower Crustacea they are called 'shell-glands' and 

 open at the base of the second maxillae (fifth segment). 



The sexes are separate in all except the barnacles, and 



