1038 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [58] 



other forms into segmentally arranged muscular somites, in this in- 

 stance becomes subservient to producing the movements of the verti- 

 cal fins. CJpon dissecting a very large si)ecimen we find that the so- 

 matic musculature is divided into bundles, which radiate for the most 

 part in a backward, upward, and downward direction, and in a semi- 

 circular series to the dorsal, caudal, and anal, into the bases of the 

 rays of which they are inserted by i)owerful tendons. There are two 

 series of these bundles, with their tendinous terminations, one on the 

 right, the other on the left side of the body, corresponding to the mus- 

 cular masses of the right and left sides of normal fishes. The tendons 

 are round and lustrous white, and pass through a mass of very tough, 

 elastic tissue, almost cartilaginous, which extends along the bases of 

 the vertical fins, long tubular openings being excavated in the basal 

 cartilage-like substance, through which the long tendons glide. In 

 this way a muscular apparatus is developed on the one side of the body 

 which o])poses that on the other, and which most effectively moves the 

 high vertical fins from side to side ; and the two series of muscles and 

 tendons inserted into the caudal move it from side to side, very much 

 like the rudder-chains of a ship move its rudder. There is also this 

 resemblance between the caudal of Mola and a rudder, that the strip of 

 compressible elastic tissue at either side of the base of the tail acts as 

 a sort of hinge, upon which this thick, rigid, truly rudder-like organ 

 swings from side to side. A slightly similar arrangement is found in 

 Ostracion, but the modification is not carried to anything like this ex- 

 traordinary degree of specialization. 



Now, what is the meaning of this modification 1 Simply this, that 

 almost the whole of the lateral muscular masses have been converted 

 into bundles of muscles terminating in tendons, the function of which 

 is to move the vertical and caudal fins. And how is this change from the 

 normal condition to be explained on embryological grounds'? If we 

 examine the developing tails of normal Teleosts we find that the caudal 

 musculature is developed from the last somites of the body; that in fact 

 almost the whole of the muscular somites of 3Iola are used up in order 

 to form the flexors of the caudal and vertical fins alone; whereas in nor- 

 mal Teleosts only a very fe\v of the terminal muscular segments of the 

 urosome are used up or transformed into the musculature of the tail 

 and the vertical fins. This, I infer, may be regarded as the final proof 

 that the tail of Mola has had a large part of its urosome aborted, as 

 already urged, so that it was necessary to modify the more anterior 

 series of muscular segments and subordinate them to the function of 

 flexing the tail. 



This metamorphosis of the myotomes of Mola into flexors of the fins 

 is doubtless due to the fact that the skin of the animal, in large speci 

 mens, is quite half an inch thick, thus constituting an almost rigid cov- 

 ering over the body, which would either induce muscular degeneration 

 or metamorphosis. 



