PARASITIC ANIMALS. 189 



live in the shells of Mollusks, such as the Pinnotheres of the 

 Oyster and Mussel. I have found other species upon Sea- 

 Urchins (Pinnotheres Melittce, a new species, upon Melitta 

 quinquefom.) The Paguri take the shells of Mollusks to 

 protect themselves ; while a vast number of Amphipods 

 live upon Fishes, attached to their gills, upon their tongue, 

 or upon their skin, or upon Starfishes. 1 The Cyamus Ceti 

 lives upon the Whale. Some Cirripeds are parasites upon 

 the Whales, others upon Corals. In the family of Ler- 

 nseans, the females are mostly parasites upon the gills or 

 fins or upon the bodies of Fishes, while the males are free. 

 Among Worms this mode of existence is still more fre- 

 quent, and while some dwell only among Corals, entire 

 families of others consist only of genuine parasites ; but 

 here again we find the most diversified relations ; for, 

 while some are constantly parasitic, others depend only 

 for a certain period of their life upon other animals for 

 their existence. The young Gordius is a free animal ; it 

 then creeps into the body of Insects, and leaves them 

 again to propagate ; the young Distoma lives free in the 

 water as Cercaria, and spends the remainder of its life in 

 other animals ; the Tsenia, on the contrary, is a parasite 

 through life, and only its eggs pass from one animal into 

 the other. But what is most extraordinary in this, as in 

 many other intestinal Worms, is the fact, that, while they 

 undergo their first transformations in some kind of ani- 

 mals, they do not reach their complete development until 

 they pass into the body of another higher type, being 

 swallowed up by this while in the body of their first host. 

 Such is the case with many Filarise, the Tseniee and Bothro- 

 cephali. These at first inhabit lower Fishes, and these 

 Fishes, being swallowed by Sharks or Water Birds, or 



2 I have found a new genus of this family upon Asterias helianthoides. 



