1060 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [80] 



flexure, but the form of the terminal ray-bearing lobes resembles v^ery 

 considerably those of Elasmobranchs, and doubtless some such form 

 anticipated the caudal of the latter. 



Their recognition of the caudal filament, or the opisthure of our nomen- 

 clature, as homologous with the dorsal lobe of A. Agassiz is also right 

 only in part, because the fin-folds are absent in this case, but present 

 in larval fishes in the lopliocercal stage; moreover, the opisthure be- 

 comes dorsal in position in the heterocercal forms very soon, but not in 

 a perfectly diphycerciil form like Chimcera monstrosa^ where it is literally 

 post-caudal, being, as a matter of fact, opisthural in position and almost 

 l^erfectly archicercal in character. 



The forms of the lobes of what may be recognized as the true caudal 

 epaxial and hypaxial fins of Chimcera agree in their general outline 

 somewhat closely with those of Elasmobranchs, but by no means exactly, 

 yet it is easy to derive the Elasmobranch caudal from the Cbimseroid 

 tail, as follows : (1.) Let the opisthure be aborted; (2) lengthen the in- 

 ferior rays anteriorly, and shorten the superior caudal rays; and (3) 

 flex the caudal axis upward, and we have the heterocercal caudal of the 

 Sharks, i^rovided the neural and haenial arches or spines are supplied. 

 At any rate, the course followed by the process of evolution in this 

 instance, which leads up to heterocercy, is clear. The diphycercal rsky- 

 bearing part of the tail of the Chimaeroids is evidently difterentiating 

 in the direction of that of the true SqtiaH, and that it represents a 

 l)hase of the evolution of the caudal in the latter is rendered all the 

 more probable from the fact that Chimcera is more primitive and more 

 embryonic in many of its characters than the latter. Let the whole 

 axial skeleton of Chimcera become more difl'erentiated, and it would 

 doubtless veer towards that of the Sharks in many of its features. 



As was remarked before, there is manifestly some stimulus to growth 

 tending to widen the hypaxial caudal lobe and lengthen its included 

 rays. That stimulus seems to us to be use and effort exerted in the 

 course of existence or during life, and through which the local activity 

 of nutritive processes is modified through the fluctuations of blood- 

 sup])ly needed in carrying on nervous and muscular action. The scull- 

 ing action of the flat tail of Chimcera in rising toward the surface would 

 necessitate the greatest exertion of effort by the lower lobe of the 

 caudal, and thTisinitiate such a differentiation as is demanded by the fore- 

 going hypothesis. For reflex activity or response to stimuli begins to be 

 manifested not in the finished metazoan organism as a whole, but has its 

 ultimate source in the histological elements or cells of which such an 

 organism is composed, an opinion with which I think it probable 

 most careful embryological students who have studied much live ma- 

 terial will agree, as it is not meant here that what are known as reflex 

 actions involving molar movements of masses of bone, muscle, &c., in- 

 clude all that is truly comprehended by reflex activity, which may in 

 reality be exhibited by the tiniest speck of protoplasm. In fa(it, all the 



