OF TUP. FISH. 199 



.apacity for speed is rwilly wonderful, ami N ii\ all .ff.-.-t.Hl by simple and beautiful modiflca- 

 inn- '' .'ii.' in.-.-luiiii-al principle, th il of the IndlMd plan-- 01 1 MM 



In all Fishes, the ]M>\MT .if progrssion li.-s in tin- wonderfully muscular tail with its 

 appended fin, and the creature drives itwlf forward by rej>eated stroked of this organ in 

 exactly the siinic inaiiii.T that a sailor urges a Itoat through the water by the back ward and 

 forward nio\.-m.-n!-. of a sinirle oar iu the stern. 



To show tin- jxiwer of this principle, I will nn-niin that. U-ing n one occasion left with a 

 juirty of friends on board a fishing-barge inn small lake, and deserted by an ill-conditioned 

 Uiatnian. who ii-fiis.il ,-ither to put us a^hor.- or tak.- us to n ln'tt-r fishing-ground, and so 

 went misanthropically home to his dinner, I called to mind the progression of the Fishes, and 

 Straightway l-came iiid<-|-n<l.-nt of the boatman After hauling up the anchor, I inserted the 

 butt end of the larg>st fishing-rod into the h>nd of the rudder no as to form an extempore 

 tiller, and by moving the rudder gently t and fro I was able to pn>j>el 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 a.s he could. 



Kven the eels and the flat Fishes, with their gracefully seri>entine movements, adopt this 

 mode of progression, though it is not so apparent as in the Pish whose bodies are less flexible, 

 and accordingly employ more for.-.- in the tail it-self. 



The fins are scar.-.-ly .-mployed 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 tin-, of a FMi are distinguished by appropriate names. As they are extremely 

 ini|>ortant in determining the series and even the genus of the individual, and as these 

 Mi.-mU-rs will be reputedly mentioned in the following pages, I will briefly de>rribe them. 



Beginning at the head and following the line of the back, we come upon a tin. called from 

 its position the dorsal" tin. In very many species then- an- two such fins, called, from their 

 relative |Mi-itions. 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 sha|>e, 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 jiortion. which starts immediately below the eye, is called the "prae- 

 operculum." and immediately liehind it comes the "operculum." Below the hitter is another 

 piece, termed, from its position, the "sub-operrnlum," and the lowest piece, which touches all 

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

 operculum, are the slender iN.n.-s. termed the " branch iostegous rays," which differ in shape 

 and number according to the kind of Fish. 



The scales with which most of tin- 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 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 littl.- tubular aperture, which is 

 v.-ry perceptible on the exterior, and < -.institutes the "lateral lin.-." 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. 



