In the Ichthyotomous sharks, Dipnoans, and Crossopterygians the segments of the pectoral limb are arranged axially, or one beyond another. This type of fin has been called archipterygium by Gegenbaur, on the theory that it represents the condition shown on the first appearance of the pectoral fin. This theory is now seriously questioned, but it will be convenient to retain the name for the pectoral fin with segmented axis fringed on one or both sides by soft rays.
Fig. 50.—Shoulder-girdle of a Hake, Merluccius productus (Ayres).
The archipterygium of the Dipnoan genus Neoceratodus is thus described by Dr. Günther ("Guide to the Study of Fishes," p. 73): "The pectoral limb is covered with small scales along the middle from the root to the extremity, and is surrounded by a rayed fringe similar to the rays of the vertical fins. A muscle split into numerous fascicles extends all the length of the fin, which is flexible in every part and in every direction. The cartilaginous framework supporting it is joined to the scapular arch by a broad basal cartilage, generally single, sometimes showing traces of a triple division. Along the middle of the fin runs a jointed axis gradually becoming smaller and thinner towards the extremity. Each joint bears on each side a three-, two-, or one-jointed branch."
In the genus Lepidosiren, also a Dipnoan, the pectoral limb has the same axial structure, but is without fin-rays, although in the breeding season the posterior limb or ventral fin in the male is covered with a brush of fine filaments. This structure, according to Prof. J. G. Kerr,[3] is probably without definite function, but belongs to the "category of modifications so often associated with the breeding season (cf. the newts' crest) commonly called ornamental, but which are perhaps more plausibly looked upon as expressions of the intense vital activity of the organisms correlated with its period of reproductive activity." Professor Kerr, however, thinks it not unlikely that this brush of filaments with its rich blood-supply may serve in the function of respiration, a suggestion first made by Professor Lankester.
FOOTNOTES:
[3] Philos. Trans., Lond., 1900.