SUBJECT INDEX — MORPHOLOGICAL SECTION 



459 



Numerous species of Teleosts have a 

 " Poison apparatus composed of spines 

 and associated glands." For references, 

 see helow this caption under Poisonous 

 fishes. 



Ichthyodorulites . Numerous isolated spines , 

 found in the Devonian and Carboniferous strata, 

 are considered to be the dorsal spines of Elas- 

 niobranchs. They constitute the sole remains 

 of these fishes. Their nature was first recog- 

 nized by Buckland and De la Beche who used 

 the term "ichthyodorulites." By Woodward 

 (p. 92), this term is restricted to thbse spines, 

 tubercles, and plates exhibiting the micro- 

 scopical structure of vascidar dentine. Nu- 

 merous genera have been proposed for these 

 isolated spines. 



_ Full bibhographic references to all genera 

 will be found in Woodward's " Catalogue '* 

 (Woodward, A. S. 1889.2, vol. ii, p. 92 et seq.) 

 and (American forms) in Hay, O. P. 1902,1, 

 p. 326. 



MEDIAN FINS 



Including the dorsal, caudal, and anal 

 fins. The median fins are believed to be of 

 greater antiquity than the paired fins. In em- 

 bryo fishes, a continuous median fringe ap- 

 pears long before the origin of the paired fins. 

 In the lower groups, Cephalochordata and Cy- 

 clostomata, the median fins are the only ex- 

 isting fins, although this condition may be due 

 to the loss of the paired fins through degenera- 

 tion. 



The median fins are considered to be the dis- 

 continuous remnants of a primitively continu- 

 ous median fringe. 



A valuable work in German, especially on 

 the embryological development of the median 

 fins, is *Schmalhausen, J. J. 1912.1. 



See also •Goodrich, E. S. 1906.1, who 

 showed that the median fins in Elasmo- 

 branchs develop essentially like the paired 

 fins. 



Through artificial breeding, the Japanese 

 have produced numerous modifications in 

 the median fins of goldfish; there are double- 

 tailed varieties, split or double anal fins, 

 etc., etc. See Bade, E. 1905.2; Matsu- 

 bara, S. 1908.1; Pouchet, G. 1870.1; 

 •Smith, H. M. 1909.2; •Storch, O. 

 1911.1; andWatase, S. 1887.1. 



In Echeneis and Remora, the dorsal fin 

 has become greatly modified to form an 

 Adhesive disk, which see. 



Morphology and development of median 

 fins chiefly the caudal fin or tail 



Most of the following references, except as 

 otherwise stated, treat of the morphological 

 nature of the termination of the vertebral 

 column in the tail. This work has been in 

 gradual expansion and more precise definition 

 of the original conception of Agassiz (J. L. 

 1833.2), who on the grounds of external sym- 

 metry recognized two types of caudal fin, the 

 asymmetrical " heterocercal " and the sym- 

 metrical " homocercal " types. 



The most recent and valuable summaries 

 of this subject, from which most of the in- 

 formation given below has been gleaned, are 

 those cited under •Whitehouse, R. H. 

 1910.1 and 1910.2. 



An older but valuable, complete discus- 

 sion of the subject is •Ryder, J. A. 1886.8. 



Other papers with further delimitations 

 and refinements in terminology are •Dollo, 

 L. 1896.2 and •Gregory, W. K. 1907.1. 



Hippocam,pus lacking a caudal fin uses 

 tail as a prehensile organ. Dufoss6, A. 

 1874.2. 



TYPES OF CAUDAL FINS 

 Protocercy. All fishes first possess a proto- 

 cercal tail in the embryonic condition, and it 

 seems safe to infer that this is the true primi- 

 tive condition, from which the more specialized 

 types have evolved* A protocercal fin may be 

 defined as one which is primitively symmetrical, 

 externally and internally, having not under- 

 gone any reduction or modification of the 

 original form. The chorda ends in a perfectly 

 straight line and the supports of the fin-rays 

 are equal in number and kind, both dorsally 

 and ventrally, so that the dorsal and ventral 

 fin-rays contributing to the caudal fin are like- 

 wise equal in number and position. The term 

 "protocercal" was first used by Wyman (J. 

 1864.2) to designate the median fin-fold con- 

 dition of the tail of the larval Raja. Agassiz 

 (A. 1878. 1,_ I, p. 123) proposed the term lepto- 

 cardial which is now considered synonymous 

 with the preceding. 



Another type of caudal fin, also perfectly 

 symmetrical both internally and externally, 

 which however is secondarily symmetrical, is 

 the gephyrocercal type defined below. 



M'Coy (Sir F. 1848.2, p. 304) first used the 

 term diphycereal to designate the symmetrical 

 caudal fin of Diplopterus. Cope (E. D. 1871.1, 

 p. 452) in ignorance of this term, coined iso- 

 cercal for the symmetrical caudal. The term 

 " diphycereal " originally used with no quali- 

 fication regarding primitive or secondary 

 symmetry, may, for all practical purposes, be 

 dismissed , in the view of Whitehouse , who 

 would however retain it "as applying to a 

 caudal fin which it may not be wise or possible 

 to refer to as protocercal or gephyrocercal." 



'* It is probably correct to say that no 

 true fishes possess a protocercal fin in the 

 adult condition ; the Cyclostomes are 

 generally credited with having proto- 

 cercal tails, but it is likely that even here 

 a degenerate condition is present. ' * 

 Whitehouse, R. H. 1910.1, p. 135. 



Median fins of Cyclostonaata 



For the caudal skeleton in Myxine and 

 Petromyzon, see •Ayers, H. & Jackson, 

 C. M. 1900.1; Cleland, J. 1886.1; Ret- 

 zius, M. G. 1892.1, 1895.2; •Schaffer, J. 

 1901.1,-2. 



The " pre-anal fin " of Myxine is a skin 

 fold, according to Lonnberg, A. J. 1902.7. 



Heterocercy. "A heterocercal caudal fin 

 may be defined as one in which the extremity of 

 the chorda is directed upwards, and as a con- 

 sequence is unsymmetrical externally and in- 

 ternally, the ventral lobe being greater than the 

 dorsal; and, moreover — and this is a most 

 important feature, to which I have not seen 

 attention drawn — when centra are formed, 

 these elements remain distinct to the end of 

 the axis." Whitehouse, P. H, 1910.1, p. 136. 



Heterocercy in Elasmobranchii 

 Chiefly, development and morphology of 

 the tail in Selachians. Ahlborn, C. G., 

 1895.2; Beneden, P. J. 1853.1, 1861.1; 

 •Dohrn, A. 1882.2 (vi); Garman, S. 

 (Chlamydoselachus) 1887.1; Haswell, W. 

 A. 1885.2; Helbing, H. 1902.1, 1904.1; 

 •Mayer, P. 1886.1; Virchow, H. 1895.3; 

 •Schmalhausen, J. Add. 1913.1. 



