18 The Exterior of the Fish 
The pine-cone fish (Monocentris japonicus) adds strong fin- 
spines to its bony box, and the porcupine fish (Diodon hystrix) 
is covered with long prickles which keep away all enemies. 
Among swift fishes, there are some in which the body is 
much deeper than long, as in Antigonta. Certain sluggish fishes 
seem to be all head and tail, looking as though the body by 
some accident had been omitted. These, like the headfish 
(Mola mola) are protected by a leathery skin. Other fishes, as 
the eels, are extremely long and slender, and some carry this 
elongation to great extremes. Usually the head is in a line 
with the axis of the body, but in some cases, as the sea-horse 
(Hippocampus), the head is placed at right angles to the axis, 
and the body itself is curved and cannot be straightened with- 
out injury. The type of the swiftest fish is seen among the 
mackerels and tunnies, where every outline is such that a racing 
yacht might copy it. 
The body or head of the fish is said to be compressed when 
it is flattened sidewise, depressed when it is flattened vertically. 
Thus the Peprilus (Fig. 10) is said to be compressed, while the 
fishing-frog (Lophius) (Fig. 11) has a depressed body and head. 
Other terms as truncate (cut off short), attenuate (long-drawn 
out), robust, cuboid, filiform, and the like may be needed in 
descriptions. 
Measurement of the Fish.—As most fishes grow as long as 
they live, the actual length of a specimen has not much value 
for purposes of description. The essential point is not actual 
length, but relative length. The usual standard of measure- 
ment is the length from the tip of the snout to the base of the 
caudal fin. With this length the greatest depth of the body, 
the greatest length of the head, and the length of individual parts 
may be compared. Thus in the Rock Hind (Epinephelus 
adscensionis), fig. 12, the head is contained 2$ times in the 
length, while the greatest depth is contained three times. 
Thus, again, the length of the muzzle, the diameter of the eye, 
and other dimensions may be compared with the length of the 
head. In the Rock Hind, fig. 12, the eye is 5 in head, the snout 
is 43 in head, and the maxillary 2%. Young fishes have the 
eye larger, the body slenderer, and the head larger in proportion 
than old fishes of the same kind. The mouth grows larger 
