ARTHOPOD A — CRUSTACEA — TRILOBITES 285 



that of the exoskeleton ; only the middle portion is unlined and 

 this alone develops outgrowths for secretion of digestive fluids 

 and for absorption. A contractile heart forces the blood through 

 the arteries to the surface of the body, whence it returns through 

 the veins or open sinuses to the heart, passing on the way 

 through the gills, where these are present. Respiration is by 

 the general surface of the body or by gills (hollow offshoots of 

 the thoracic walls or of the thoracic or abdominal limbs). The 

 young usually passes through a series of larval stages, — the 

 nauplius, zooea and mysis stages. Through successive molts 

 the young animal increases in length by the addition of new 

 segments anterior to the telsonic region. 



Abundant as fossils. 



Derivation of name. — Latin crusta, a crust, referring to the 

 hard, crust-like, calcareo-chitinous skeleton completely inclos- 

 ing the animal. 



The class Crustacea is divided into the following sub-classes : 



Page 



1. Trilobita 285 



2. Phyllopoda 299 



3. Ostracoda 303 



4. Copepoda 304 



5. Cirripedia 305 



6. Malacostraca 3o5 



7. Stomatopoda 308 



Sub-class i, Trilobita 



Type of the sub-class, Triarthrus (Fig. 126). 



Triarthrusis much more closely allied to Apus (see p. 299) than 

 to the crayfish, or any other living form, but lacks the peculiar 

 carapace and probably the long, anal processes of the former. 

 These processes, however, are known to be present, extending 

 backward from the ventral surface of the abdomen, in Neolenus 

 serratus of the Burgess shale (mid-Cambrian) of British Colum- 

 bia. The carapace is lacking in many species of the order Phyl- 

 lopoda, to which Apus belongs ; this is true of the fresh water 

 shrimp, Branchipus, and of the brine shrimp, Artemia, but in 



