124 FISH—CULTURAL ASSOCIATION. 
onists whose attacks would be disastrous. The tunny or horse- 
mackerel, Orcynus thynnus, other swordfishes, and sharks are its 
only peers in size, and of these the sharks are probably its worst 
foes. 
Capt. N. E. Atwood exhibited to the Boston Society of Nat- 
ural History, December 7th, 1864, the lower jaw of a large 
shark, taken at Provincetown, Mass., in whose stomach nearly 
the whole of a large swordfish was found. Some ten or twelve 
wounds were noticed in the skin of the shark, giving an idea of 
the conflict. The shark was doubtless Galeocerdo tagrina. 
Couch was told by a sailor that he had watched with interest 
the anxious motions of one as it was followed closely and rapid- 
ly in all its turnings by a blue shark. Twice did it leap above 
the surface to escape the near approach of its pursuer, but with 
what success at last the observer had no opportunity of knowing. 
Mr. John A. Thomson states that the billfish (probably Zetrap- 
turus albidus) is their special enemy. Billfish, six to twelve feet 
long, appear about the last of the season, and their appearance is 
a signal that the swordfish are about leaving. 
INVERTEBRATE PARASITES OF THE SWORDFISH. 
Aristotle thus explains the leaping movements of the fish: 
“The tunny and the AX7p/za suffer from the cestrus at the rising 
of the dog-star, for both these fish at this season have beneath 
their fins a little worm which is called cestrus, which resembles 
a scorpion and is about the size of aspider; they suffer so much 
from this torment that the Xzphzas leaps out of the sea as high 
as the dolphin, and in this manner frequently falls upon 
ships.” 
This description of the parasite is somewhat vague; yet it is 
evident that allusion is made to one of the Lerneans or gill-lice, 
little crustaceans remotely resembling crabs and lobsters, which 
attach themselves to the gills and skin of many kinds of fishes, 
sucking the blood from their veins, and often causing death ; 
dreadful to their victims as was their namesake, the fabled Ler- 
nan Hydra, to the Argives of old, and not to be destroyed by 
any piscine Hercules and folaus. 
In one of the early volumes of the Philosophical Transactions 
