. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. LOCOMOTION OF FISHES. 253. nf h>ri>inotive act, ^li. C-\XXJ. flexion in e o, acts backward, in tlic direction of o e; havinf; reached the point o, it is again forciljly bent in the line o e causing an impulse on the centre of gravity in c b, parallel to o e; if the tAvo forces c h and c h acted simul- taneously, we should obtain the resultant c f; but, as they do not, the point c will not move exactly in the line c f, but in a curved line, evenly between d ey"and a line drawn parallel to it tln-ough /(. The fi
. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. LOCOMOTION OF FISHES. 253. nf h>ri>inotive act, ^li. C-\XXJ. flexion in e o, acts backward, in tlic direction of o e; havinf; reached the point o, it is again forciljly bent in the line o e causing an impulse on the centre of gravity in c b, parallel to o e; if the tAvo forces c h and c h acted simul- taneously, we should obtain the resultant c f; but, as they do not, the point c will not move exactly in the line c f, but in a curved line, evenly between d ey"and a line drawn parallel to it tln-ough /(. The fish being in motion, the tail descril)es the arc of an ellipse; whereas, if it were station- ary, it would describe the arc of a circle. The power of varying the position and ex- panse of the tail-fin during the side-strokes complicates the ])roblem ; its plane may be perpendicidar to the stroke's direction, and its expansion greatest at the begiiniing of the stroke, as in a i ; and it may be oblique to the direction of the rest of the stroke, as in e o, with contraction of the surface. It must, further, be considered that the water having been set in motion by flexion in one direction, produces, when meeting the tail moving in the opposite direction, a resis- tance proportional to the sum of the squares of the two velocities. The shape of the caudal fin varies nuich in fishes, according to the kind and degree of motion required : in the imprisoned embryo, or newly-hatched fry, in the long and slender undulating eel, in the sluggish Lepidosiren, the vertelDra; continue to the end of the body in a straight line, distinct, and decreasing to a point; and the tail is bordered above and below by a vertical f(.)ld of skin; terminating either in a point, as in fig. 100, or obtusely. Such fold or fin is symmetrical, but not ' homocercal.'' The vertical folds deepen; at first, in some Plagiostomes, e. g., equably, forming a terminal lobe ; then excessively, in the lower or ha3mal fold, w
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