104 



SCIENCE, 



[YOL. VI., No. 131. 



native element to discover that the great dorsal and 

 anal fins are almost its sole organs of propulsion. In 

 th3 figure, both are shovs^n with their tips thrown over 

 to the right side. This movement is synchronous in 

 these great fins; that is, both strike in the same di- 

 rection at the same time. But the most remarkable 

 feature about them is, that they are twisted into the 

 form of the blade of a propeller-wheel at the com- 

 mencement of each stroke, as shown (not very clearly) 

 in the figure, where the foreshortened dorsal is indi- 

 cated in the condition which it presents at the moment 



MOLA ROTUNDA [viewed from above, -^^-z nat. size). 



when the stroke to the left is about to begin. The 

 direction of the pecuhar twist of these fins is also re- 

 versed at each successive stroke, in opposite direc- 

 tions, so that a most effective propelling apparatus 

 exists, which it would be somewhat diflBcult to imi- 

 tate by a mechanical device. This contrivance is ren- 

 dered all the more effective in consequence also of 

 the great width of the fish, by which great stability 

 is secured. 



A very peculiar soft oval ring of dermal tissue 

 around the bases of the dorsal and anal, renders 

 them readily movable by means of the tendinous 



terminations of the muscles which are inserted into 

 the strong bases of their rays. A similar band of 

 soft dermal tissue is found on both sides of the base 

 of the broad, crenulated, rigid tail, which is moved 

 on its base just as the rigid rudder-blade of a vessel 

 is moved by means of the rudder-chains. In the case 

 of Mola, however, the single pair of rudder-chains as 

 found in a vessel is replaced by a single series of 

 tendons on either side of the median line, running 

 parallel, and passing through round canals in the soft 

 elastic base of -the caudal, and inserted into the bases 

 of the caudal rays. 



The office of the curiously modified caudal of Mola 

 seems to be solely that of a rudder, no vibration 

 whatever of the tail being apparent when the animal 

 is swimming in a straight line. Only when the crea- 

 ture wishes to turn is it deflected from the median 

 line at its tip ; and even then its apex sweeps through 

 an arc of not more than twenty-five degrees at most, 

 usually much less. 



The pectorals are comparatively small, and, in 

 swimming in a straight line, are extended horizon- 

 tally and kept rigid and motionless by the animal, as if 

 merely for the purpose of keeping the body vertical. 

 When swimming with the sides of the body at an 

 angle with the plane of the horizon, one or the 

 other of the pectorals is folded closely against the 

 side. 



It thus becomes evident that Mola uses almost 

 exclusively the dorsal and anal fins as locomotive 

 appendages, and that the tail and pectorals act re- 

 spectively as the rudder and as balancers, and take 

 no active, but only a very subordinate, share in effect- 

 ing the creature's movements. Sven the rudder-like 

 tail may i^^e said to be defective; for it was noticed 

 that the fish, when first placed in the pool, was un- 

 able to turn quiclily enough to avoid bumping its 

 snout against the stone walls at the angles of the 

 enclosure. 



The organization of this creature is probably the 

 most remarkable of any encountered amongst the 

 Teleostei. The body is incased in a very thick in- 

 flexible skin, and all of the lateral musculature has 

 become subordinated to the office of flexing the 

 vertical fins. In fact, so extreme has been the 

 muscular specialization, that the myotomes have 

 been entirely suppressed, and the lateral muscular 

 masses developed into long muscles, ending in 

 tendons. 



No less remarkable has been the mode in which 

 the tail has been developed through a larval form, 

 probably provided with a lophocercal tail, which has 

 already disappeared in the stage represented by Mola- 

 canthus. Mola, in fact, is the best-known example of 

 a type having a gephyrocercal structure. No less ex- 

 treme is the modification of the cerebro-spinal nervous 

 system; for, as a result of the muscular specialization 

 already noticed, the medulla spinalis has been exces- 

 sively abbreviated. We are, accordingly, forced to 

 admit, that Mola is the most extremely modified te- 

 leost in existence, notwithstanding the fact that a 

 great many very singular deep-sea forms have been 

 described within a very recent period. 



The great advantage of having the large pools of 

 salt-water enclosed by heavy stone walls also becomes 

 very apparent in the instance of Mola, and shows 

 how valuable they may become as aids in the study 

 of the habits and modes of life of such large marine 

 forms as dolphins, porpoises, sharks, swordfisb, etc., 

 — types in which the Fish commission is interested, 

 as affecting the abundance of certain of the food- 

 fishes. 



JoHJsr A. Rtdee. 



