Chap. III.] 



CRUSTACEA. 



6 9 



while the extreme modification is seen in Argulus, a 

 common parasite on the stickleback. 



3. In the Ostracoda the carapace forms a com- 

 pletely bivalve shelly covering for the body, the 

 abdominal region of which is greatly reduced. Cypris 

 arid Cythere are examples. 



4. Although the Cirripedia are, when adult, 

 greatly altered by their fixed or parasitic habit, they 

 leave the egg as Naupliiform larvae ; these become 

 attached by their anterior 



ends, and enclosed in a 

 sac-like mantle formed by 

 the integument ; this 

 either remains soft, as in 

 Alcippe, which lives in 

 cavities, and is thereby 

 protected, or undergoes 

 calcification, when a 

 greater or less number of 

 plates become developed. 

 The anterior region is 

 either broad, as in the 

 acorn shell (Balanus), or 

 drawn out into a stalk, as 

 in the barnacle (Lepas). 



5. The Ceiitrogo- 

 iiida, or, as they are 

 often called, Rliizo- 

 cepliala, are usually 

 found on the bodies of 

 higher Crustacea after the 

 nauplius stage is passed. 

 They are endoparasitic, 



and, later on, form a sac without limbs on the outer 

 surface of their host's body. To this group belong 

 Peltogaster and Sacculina. 



B. The Malacostraca have almost constantly 



Fig. 29. Squilla mantis. 



