MINOR ABNORMALITIES— RELATING TO FINS 55 



Lcpidosiren, an instance in which the extremities of both pectorals underwent some degree of 

 branching (Goeldi 82). 



Barfurth's Petromyzon larva with trilid tail should again lie noted (see p. 36 of this work). 



Causation. Certain cases of supernumerary fins are due to the persistence of an embryonic 

 twin rudiment in this form. The instances recorded by Mazzarelli (164) an( i Gervais (80) appear 

 to be of this kind, and possibly also those given by Rennie (80S). It is by no means uncommon to 

 find among broods of young trout towards the end of the embryonic stage examples of parasitic twins, 

 in which one or both of the pectoral fins are the chief remaining structures. Lereboullet's (143) 

 work on the pike provides an occasional example of this kind observed during actual development. 



The doubled dorsal fin which I observed in a recently hatched trout embryo could best be 

 accounted for as being due to a healed-up partial mesodidymus. 



For other cases the somewhat theoretical explanation may be advanced (Rennie SOS), that there 

 has been an attempt at independent development on the part of germ-cells that have somehow 

 wandered or become separated from their proper locality. Or one may speak simply of hypertrophy 

 with excess of the particular fin-forming embryonic material, and draw comparisons with examples of 

 malformation by excess in other structures, e.g. digits (M'Intosh 155). 



Certain instances are clearly due to supra-regeneration after removal or to undue growth 

 following injury or irritation. This applies particularly to the Dipnoan cases and, according to 

 Boulenger (30), to that recorded by him in the plaice. 



The goldfish races provide examples of great germinal variation, the extreme forms of which 

 have been preserved and accentuated by selection. 



(b) Defect. Examples : 



(1) Absence. The following cases may be noted: Cyprinus, with tail fin awanting, Otto (178 

 p. 133), Fiebiger (6S), Nusbaum (173) ; Esox lucms, with false tail fin (Hofer 96) ; Abramis blicca, 

 without pelvic fins (Brindley 32 and probably same specimen Lowne 14-0 p. 112) ; Acantltla* vulgaris, 

 without second dorsal and caudal fins and with tail deformed (Grosser u. Przibram 85) ; goldfish 

 races with dorsals awanting (Tornier 24-9) ; Amciurus natalis without trace of pelvic fins (Eigenmann 

 and Cox 62). See also p. 61. 



(2) Defective development. Goldfish races with various fins extremely reduced, Tornier (249) ; 

 Cyprinus with reduced dorsal, Otto (178 -p. 266) ; Raia clavata, showing notch between the head and 

 the anterior edge of the pectoral fins, so that the latter sticks out as a horn on either side of the 

 former, Day (54 II. PI. CLXXI. 2), Traquair (253), Johnstone, 1 Vaillant (262), Williamson (270 

 p. 54); cyclopic Myliobatis, with similar horns, Paolucci (181); misshapen spines and rays in 

 the anal fin of Scbastcs, Jaquet (111); deformed caudal fin in Motella, Jaquet (112). 



Duncker (59 375-378) refers to a number of minor abnormalities in the fins of the genus 

 Pleuronectes, e.g. reduction in size of the pelvic fins ; blanks in the series of dorsal and anal fin rays ; 

 rays developed only in their proximal or distal parts ; doubled rays ; fusion of distal ends of fin 

 rays ; fusion of edge of right pectoral fin with the adjacent skin. 



Numerous instances are on record of flatfish with the anterior extremity of the dorsal fin 

 ending abruptly, so that a notch occurs between this fin and the head. The condition is usually 

 associated with abnormal pigmentation of the under side, and often also with incomplete migration 

 of the eye belonging to the side in question (see p. 56). 



Causation. No single factor is sufficient to explain all cases of defective or absent fins. It 

 seems necessary to assume the following as possible causes: (1) injury or removal followed by 

 healing, and in some cases by partial regeneration ; (2) arrest of development due to external 

 causes acting during embryonic life, or in the early stages of growth; (3) spontaneous germinal 

 variation, or reversion. Probably the first four cases cited under Absence belong to the category of 

 injuries, while the Acanlhias provides an example of induced arrest, and the remainder are mainly 

 due to the third of the causes named above, though some of them may be referable to the others. 



1 Report, 1905, Lancashire Sea Fisheries Committee, p. 188. 



