STRUCTURE OF THE FISH. 189 



capacity for speed is really wonderful, and is in all effected by simple and beautiful 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 virges 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 misanthroplcally home to liis dinner, I called to mind the progression of the Fishes, and 

 straightway became independent of the boatman. After hauling up the nnchor, I inserted the 

 butt end of the largest fishing-i'od into the head of the rudder so as to form an extempore 

 tiUer, 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 names. As they are extreiuely 

 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 fii'st and the second dorsal fins. The extremity of the body is furnished 

 with another fin, popidarly called the tail, but more cori'ectly 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 tiie under surface and in front of the vent is called the 

 abdominal fin, and that whicli is nlso on the lower surface, and between the vent and the tail, 

 is known by the name of the ''annT' fni. All these fins vary extremely in shape, size, and 

 position. 



The gill-cover, or operculum as it is teclmically 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 "prse- 

 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 all 

 the three above it, is called the " iuter-operculum." Below the chin and reaching to the sub- 

 operculum, are the slender bones. Termed the " branchiostegous rays,'' wliich differ in shape 

 and number according to the kind of Fish. 



The scales with which most of tlie Fish are covered are very beautiful in structure, and 

 are formed by successive lamina', 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 jiass 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 peiTnit 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 detennining 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. 



