1040 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [60] 



It now remains for uie to suggest that the true larval condition of 

 Mola should be sought for amongst the surface trawlings taken during 

 marine explorations, and I will venture to say that when that larva is 

 found it will very probably ditfer as much in general appearance from 

 Ostracion hoops as the latter differs from Molacanthus or Mola. It is 

 very probably provided with some rudiment which represents the tail of 

 normal fish larva?. The eggs of Mola are very j)robably jjelagic, the 

 larvae having the same habit. 



X. — Discussion of the serial homology and the influence of 



HEREDITY ON THE DEVELOPMENT OF CONTINUOUS FIN FOLDS. 



The Ichthyopsida or Anamnia constitute the lowest portion of the 

 vertebrate phylum. In contradistinction to the more developed types, 

 they possess, almost without exception, at some stage or other, an 

 eradiate dorsal and ventral fold of epiblast, which serves as an organ 

 of projmlsion through the medium in which they usually live, either 

 temporarily or permanently. This is characteristic of the group, from 

 Branchiostoma to the highest Batrachians, and all the exceptions which 

 are known amongst existing forms are readily explicable on the ground 

 that these have specialized modes of development, which either abbrevi- 

 ate the latter process or induce ])recocious degeneration of this partic- 

 ular structure. The entire i)iscine series possess in some form or other 

 this mesial propelling organ, supported in the adult by simple or dichot- 

 omous segmented osseous, or by cartilaginous rays, or by fibers. In 

 contrast to this series the median fins are more or less transitory or 

 larval in the Amphibian series, and uni)rovided with rigid axial sup- 

 porting organs. 



The only Vertebrate which still retains a more primitive system of lo- 

 comotive organs than the median system of fin-folds, characteristic of 

 the larvae of the rest of the Ichthyopsida, is Branchiostoma while it is 

 still in the archicercal or vermiform stage, when most of its epidermis 

 js still clothed with vibratory appendages or cilia. 



The lophocercal eradiate stage of development of the azygous epi- 

 blastic — really epidermic — fold on the dorsal and ventral aspects of the 

 body seems, therefore, to be eminently characteristic of most fish-like 

 forms during au early stage of their growth. The multiradiate diphy- 

 cercal condition, which replaces it by a process of natural evolution in 

 Ceratodiis and Froioptcrus, is ])rimi+ive, and ])ermauently approached 

 and represented only by the numerous distal branches into which the 

 rays of many Teleosts subdivide atjheir tips, where traces of the dis- 

 tinct primitive fibers or horny filaments are often clearly marked, even 

 in the most specialized forms, such as /Svomher and Xiphias^ or the Mack- 

 erel and Sword fish, along the margin of the caudal fin. 



Ceratodiis and Protoptems are examples of an imperfect embodiment 

 of the protopterygian stage. In these some of the primitive horny 

 fibers or fin-rays have fused together to the nuinber of as many as three, 



