208 ELEMENTS OF COMPARATIVE ZOOLOGY. 



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

 dorsal in position, but may he either in thorax or abdomen. 

 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 we 

 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 there are excretory organs (nephridia) 

 which 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); 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 the 

 ducts of the reproductive organs open to the exterior in 

 the thoracic region, never in the abdomen. In almost all 

 forms the eggs are carried about by the mother until they 

 are hatched. In almost all the lower Crustacea the young 

 escapes from the egg in a very immature condition, known 

 as a Nauplius, a name given years ago under the belief 

 that it was an adult. The nauplius has an unsegmented 

 body, a single median eye, and only three pairs of appen- 

 dages antennulae, antennae, and mandibles the anten- 

 nulae being solely sensory, while antennae and mandibles 

 are used in both swimming and eating. In the higher 

 Crustacea the nauplius stage is passed in the egg, and the 

 young hatches in a more advanced condition sometimes 



