1066 



SCANDINAVIAN FISHES. 



rlie vent and advance along ciicli side of the belly". 

 Vet this structure iwis undergone i;reat development, 

 retaining meanwhile tlie axial parts of the paired fins 

 to such an extent tliat mor])hological comparison may 

 have recourse to the; Elasmol)raiichs in (]uest of starting- 

 points for the inter|)retation of the ai-ms and legs of 

 the highei' vertebrates. From embryology HALKoru'' 

 and WiEDERSHETM'' learnt — as 'riiA<'iii:n'' and Mivaht'" 

 had previously discovered by researches in comparative 

 anatomy — that the primordial parts in the skeleton of 

 the paired tins are the basal portions nf the cartilagi- 

 nous rays (primary radialia), whicii grow inwai-ds to 

 form the groundwork of the shoulder-girdle and the ])el- 

 vis, and in their outward collocation, radiating unilater- 

 ally from a basal part, are split in a transverse direction 



a still surviving Lung-tish'. 'flic a\ial ]iarts wliich iiave 

 liecn retained in the paired tins of tlie Ii\ing Elasmo- 

 braiichs have been named by OEiiENiiAii! the aiiterior, 

 middle, and posterior pteryyial parts (pro-, ))ieso-, and 

 metapteryginm). In some forms, as in the Rays, the 

 Xotidanoids, and some Dthei' Sharks, all these parts 

 share in the articulations of tlie pectoral tins, in others 

 onl}' two of them or e\eii oiih' one, as the projjtery- 

 giuni and a i)art of the metapteryginm in (Jliimmra 

 (fig. 2114, pj) and );//,), and the m('ta])terygium in 

 Sri/iniHfs. The last-mentioned jieculiai'itA' may also be 

 observed in the ventral tins. In the ( 'hondrosteans and 

 Teleosts we have seen how these parts gradually dis- 

 appear during the development (if the normal piscine 

 tyjjc. In the ventral fins of all male Elasmobranchs, 



"" fl]il^'i~ 



I'fpd 



,,a3.-ew«ffl^S^?f;^^"' 



<\U- nil. I li-fl vi-iitrul liii ..f a Chimojra 



Homti- 



sa, (f. -J luit. .'iize. 



Fig. 20.5. Left siile <.f the |iulvic 



b, undivided basal cartilage of (lie veiitnil tin. iiiiswcriiig to tlie propterygiiiin and niesopterygium ; for, olitiirator foramen, at the .side of 

 whicli appear two fenestra?, closed witli niciiibrane, in the pelvic cartilage; /nl, iliac part; pis, ischiadic part; /)/), ]mbie part; prp, pra'piihic 

 part, Herrated cartilage or anterior copiilatory organ of ihe male; ptpd, ptenjgopodium of the male, inetapterygoid structure, consisting of: 

 mti, inner inela]iterygoid cartilage; m(, and mt^, inner (proximal) and outer (distal) inetapterygoid parts, the latter divided into three bran- 

 ches, the outer superior {ar). tlie inner superior (xi) .ind tlie inner inferior (ii). 



and multiplied within tlie tin itself. That the prim- 

 ordial arrangement, however, here as in the diphycercal 

 caudal fin, was bilateral, with rays on each side of an 

 axial part, ajjpears from such forms as Pleuracanthus, 

 ■A primitive Shark of the Permian fauna, or ('eratodiis. 



on the other hand, the metapterygium is prolongated. 

 into free appendages (tig. 295, ptpd), which may serve 

 as (H-gans of motion, but are propei'ly copulatory organs. 

 The male (-hitUcvras are furnished with another ap- 

 pendage, possessing the same function, a uKibile, sickle- 



" Gf. Smitt: Ur dc liogre iljiir/'ns lUvfcktiiiyslii^^toria, pp. "240 .iiel 



'' Comparative /imbryologij, vol. II, pp. 492, cctt. 



' Das Gliedmas.ienskelet der Wirbelthiere, .lena 1892. 



^ Median and paired Jinfi, Trans. Connect. .Vead.. vol. Ill (1877). 



' On the Fins of Elasmobranchii, Trans. Zool. Soc. Loml.. vol. X. 



-'' Appending a reference to the stnictiire of the ]iiiirccl fins i 



mtnitii.'.; 1 expressed in 1873 the above-cited opinion, wliicli 



WlKUKRSHEIM (1. 



p. l.S) calls "Die TuACHF.n-Mlv.MiT-BAi.FOlili-HASWELI.-DnnnN'sche L'-bri'". 



