588 The American Naturalist. [July, 
stitute the chief source of their nourishment. Having no jaws, these 
leeches can neither bite nor suck blood, but swallow the worms entire. 
O. octonaria is one of the largest leeches known. The dimensions of 
one specimen found by the author is given, length 270 mm., width 14 
mm., depth 10 mm. 
Dr. Oka adopts the classification of R. Blanchard (1894), and shows 
the systematic position of Orobdella in the following synoptical table: 
Ordo Hirudinea. 
a. Subordo. Rhyncobdellae. 
b. Subordo. Arhyncobdellae. 
1. Fam. Gnathobdellidae. 
Aquatic: gen. Hirudo, Haemopis, ete. 
Terrestr. gen. Haemadipsa, Xerobdella, Mesobdella. 
2. Fam. Herpobdellidae. 
Aquatic: gen. Herpobdella, Dina, Trocheta. 
Terrestr: gen. Cylicobdella, Lumbricobdella, Orobdella. 
(Journ. Coll. Sci. Imp. Univ. Japan., Vol. VIII, Pt. 2, 1895.). 
The Origin of Tail-forms.—The use and meaning of the asym- 
metrical types of tail-fin which are so commonly met with among 
fishes—e. g., the upturned tail of the shark and sturgeon, and the 
downwardly extended fin of the flying-fish, are explained by Dr. F. 
Ahlborn by comparisons founded on experience in rowing. Every 
tyro knows the consequences which ensue if he holds his blade too 
obliquely in the water. If the upper edge is inclined too much towards 
the stern of the boat a brisk pull upon the handle results in the blade 
jumping out of the water ; if, on the other hand, the blade is inclined 
too much in the pen direction, it digs into the water and the oars- 
man “catches a crab.” The relevance of these illustrations is found 
in the fact that the skeletal support of the asymmetrical tails of fishes — 
is generally such that either the upper or lower border of the fin is 
more resistant to the pressure of the water than the opposite border, a 
fact which causes the fin in action to assume an oblique instead of a 
vertical position. The result of such a disposition is that in those cases 
where the upper part of the tail is stiffer than the lower, the tail in 
locomotion is driven upwards, as the oar is driven out of the water 
(heterocereal tail of shark and sturgeon); while in cases where the 
lower part of the tail is firmer than the upper, the tail tends, in action, 
to assume a lower position than the rest of the body (flying- fish). The 
body of the animal, in fact, is made to swing vertically about a hori- 
‘zontal axis running through the center of gravity: in the first group 
