The Exterior of the Fish 19 



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 Aiitigonia. Certain sluggish fishes 

 seem to be all head and tail, looking as though the body by 

 some accident had been omitted. These, Hke 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 (Lopliius) (Fig. 11) has a depressed body and head. 

 Other terms as truncate (cut oft" 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 2f 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 

 r head. In the Rock Hind, fig. 12, the eye is 5 in head, the snout 

 i is 4f in head, and the maxillary 2|. Young fishes have the 

 e eye larger, the body slenderer, and the head larger in proportion 

 t than old fishes of the same kind. The mouth grows larger 



