STRUCTURE OF THE FISH. 
189 
capacity for speed is really wonderful, and is in all effected by simple and beantifnl modifica- 
tions of one mechanical principle, that of the inclined plane or screw. 
In all Fishes, the power of progession lies in the wonderfully muscular tail with its 
appended fin, and the creature drives itself forward by repeated strokes of this organ in 
exactly the same manner that a sailor urges a boat through the water by the backward and 
forward movements of a single oar in the stern. 
To show the power of this principle, I will mention that, being on one occasion left with a 
party of friends on board a fishing-barge in a small lake, and deserted by an ill-conditioned 
boatman, who refused either to put us ashore or take us to a better fishing-ground, and so 
went misanthropically home to his dinner, I called to mind the progression of the Fishes, and 
straightway became independent of the boatman. After hauling up the anchor, I inserted the 
butt end of the largest fishing-rod into the head of the rudder so as to form an extempore 
tiller, and by moving the rudder gently to and fro I was able to propel the barge in any 
direction and to any distance. We thus traversed the lake at our pleasure, drove the barge 
ashore at its further extremity, and left the boatman to find it and take it back as he could. 
Even the eels and the flat Fishes, with their gracefully serpentine movements, adopt this 
mode of progression, though it is not so apparent as in the Fish whose bodies are less flexible, 
and accordingly employ more force in the tail itself. 
The fins are scarcely employed at all in progression, but are usually used as balancers, 
and occasionally to check an onward movement. Before proceeding further, I may mention 
that all the fins of a Fish are distinguished by appropriate naiqes. As they are extremely 
important in determining the species and even the genus of the individual, and as these 
members will be repeatedly mentioned in the following pages, I will briefly describe them. 
Beginning at the head and following the line of the back, we come upon a fin, called from 
its position the “ dorsal” fin. In very many species there are two such fins, called, from their 
relative positions, the first and the second dorsal fins. The extremity of the body is furnished 
with another fin, popularly called the tail, but more correctly the caudal fin. The fins which 
are set on that part of the body which corresponds to the shoulders are termed the “pectoral” 
fins ; that which is found on the under surface and in front of the vent is called the 
abdominal fin, and that which is also on the lower surface, and between the vent and the tail, 
is known by the name of the “anal” fin. All these fins vary extremely in shape, size, and 
position. 
The gill-cover, or operculum as it is technically called, is separated into four portions, 
and is so extensively used in determining the genus and species that a brief description must 
be given. The front portion, which starts immediately below the eye, is called the “prae- 
operculum,” and immediately behind it comes the “operculum.” Below the latter is another 
piece, termed, from its position, the “ sub -operculum,” and the lowest piece, which touches al] 
the three above it, is called the “ inter-operculum.” Below the chin and reaching to the sub' 
operculum, are the slender bones, termed the “ branchiostegous rays,” which differ in shape 
and number according to the kind of Fish. 
The scales with which most of the Fish are covered are very beautiful in structure, and 
are formed by successive laminae, increasing therefore in size according to the age of the Fish. 
They are attached to the skin by one edge, and they overlap each other in such a manner 
as to allow the creature to pass through the water with the least possible resistance. The 
precise mode of overlapping varies materially in different genera. Along each side of the Fish 
runs a series of pores, through which passes a mucous secretion formed in some glands 
beneath. In order to permit this secretion to reach the outer surface .of the body, each scale 
upon the row which comes upon the pores is pierced with a little tubular aperture, which is 
very perceptible on the exterior, and constitutes the “lateral line.” The shape and position 
of this line are also used in determining the precise position held by any species. In 
comparing the scales taken from different Fishes, it is always better to take those from the 
lateral line. 
The heart of the Fish is very simple, consisting of two chambers only, one auricle and one 
ventricle. The blood is in consequence cold. 
