148 SYSTEMATIC ZOOLOGY 



each individual has early in its development, and sometimes 

 throughout life, an ordinary simple eye, unpaired, on the median 

 line of the head. In addition there is a pair of compound eyes, 

 which consist of a large number of parts, each part a structurally 

 complete single eye. These compound eyes are sometimes ses- 

 sile, i.e. on a level with the skin or shell ; in other cases they are 

 raised on the tip of a stalk. 



There is much variation in the number of segments in the 

 body, but the five anterior segments are always fused together 

 to form the head, which in addition to the eyes has two pairs of 

 feelers, or antennae, and three pairs of mouth parts. The shape 

 and structure of the body present the widest variation, owing to 

 the very diverse habits of life in this group. While the majority 

 are free-living, many are more or less parasitic, generally ex- 

 ternal parasites, and present many stages of degeneracy ; some 

 are parasitic on other Crustacea. Sometimes only one sex is 

 parasitic, and there may be great sexual dimorphism. They are 

 for the most part carnivorous animals, but some live on dead 

 organic matter, and a few feed on plants. Many minute species 

 are very transparent, and such as live in salt water are often 

 highly phosphorescent. Some Crustacea have a direct develop- 

 ment, but the majority have a larval stage very different from 

 the adult. This subtype is divided into two classes. 



CLASS I. ENTOMOSTRACA 



The Entomostraca (Gr. eWo/uo?, cut up, and oarpaicov, shell) 

 include the less highly developed Crustacea, simple in structure 

 and small, though the largest may attain a length of eight centi- 

 meters. There is the greatest variation in the general shape of 

 the body and also in the number of segments it contains, even 

 closely allied genera being widely divergent in this respect. 

 The simple unpaired median eve of the larva usually persists 

 throughout the life of the individual, and there is in addition a 

 pair of compound eyes as a rule. Often there grows backward 

 from the head a fold of the integument, forming a sort of cover- 

 ing for the whole or a part of the rest of the bodv. Such a 

 shieldlike plate is called a carapace; sometimes there are two 

 folds, forming a bivalve shell, like that of a mussel or clam, 



