MANUAL OF NATURAL HISTOBY. 305 



united ; operculum of four valves ; base cal- 

 careous or membranous. Living attached to 

 stones, shells, &c. 

 Family. — Whale -Barnacles (Coronulidae). Shell 

 composed of six, distinctly separate, porous 

 valves ; operculum of four thin valves united 

 to the margins of the aperture by a mem- 

 brane ; no distinct base. Parastic on whales, 

 &c. 



FISH-PARASITES. 



The Epizoa or Fish-Parasites are either Worm- 

 like or Crab-like: the higher forms, or Carcinoid 

 type, comprehending the " Siphostomes" of Latreille, 

 or those suctorial Crustaceans with rudimentary 

 legs, sometimes in the form of little swimming- 

 paddles, and sometimes shaped like hooks, to en- 

 able them to retain their hold on the skins of fishes 

 which most of them infest ; the lower forms, or 

 Helminthoid type, partly comprising the Polycotyla 

 of Blainville, and sometimes included with the 

 Parenchymatous Parasites of Cuvier. 



The shapes these singular animals assume in the 

 various genera are frequently most grotesque, and 

 seem to resemble higher organisms in an embryonic 

 state. In the young stage, they are subject to seve- 

 ral metamorphoses, often casting their skin ; in this 

 condition they swim freely about, but after they be- 



