3t8 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



as a collector for branches from the dorsal stems. The action of both 

 muscles is on the fins, and the plexus is probably necessary to give 

 the various parts of the muscle simultaneous contraction and so pro- 

 duce effective action on the dorsal, anal, or ventral fins. 



The Pectoral Fin Muscles. — In the Teleosts the muscles of the 

 pectoral fin have been described as consisting of two layers, an 

 abductor and an adductor layer, each being again separated into a 

 superficial and deep layer. At first sight the arrangement in 

 Amiurus appears to depart somewhat widely from this type, but fur- 

 ther investigation shows that the departure from it is merely appar- 

 ent, the true relations of the muscles being obscured by the excessive 

 development of the coracoid, whereby one portion of the abductor 

 profundus appears to lie on a different surface of the arch from the 

 other portion. The explanation of this has already been given in 

 connection with the description of the muscle. With regard to the 

 innervation of these muscles it is found that, as in higher animab, 

 there is a well marked plexus, consisting of the first three spinal 

 nerves. Following out the line of argument hitherto adopted, what 

 conclusion is reached 1 Simply that the pectoral fin, or at any rate 

 its musculature, is derived from three myomeres. It does not appear 

 that this conclusion can be escaped. Dohrn, on embryological 

 grounds, comes to the same conclusion, 1 i.e., that the pectoral is 

 formed by the accrescence of several segments. This is, of course, 

 in direct opposition to the Gegenbaurian theory, which seems now to 

 have received its quietus, having been founded on the structure of 

 the fin in an exceptionally modified form, and not representing in the 

 least the original features. 



Another fact may be here pointed out. The muscles of the fin all 

 lie on the external, inferior or posterior surface of the pectoral arch. 

 This would tend to indicate that the arch, or a part of it, is of the 

 nature of a rib, or is formed by the union of several rib-like struc- 

 tures. The manner in which certain muscles are inserted into it, 

 and others take their origin from it, supports this theory. Perhaps, 

 with Gegenbaur, one can after all, though in a different sense, refer 

 the pectoral girdle to the type of a branchial or similar arch, consid- 

 ering the arches of the other segments of which the fin is composed 

 either to have united with this one or to have entirely aborted. 



The Pelvic Fin Muscles. — Similar remarks apply to the pelvic fin. 



1 Dohrn. — Mitth aus d. Zool. Station zu Neaple, Vol V., 1S84. 



