THE MYOLOGY OP AHIURUS CATUS. 3'i9 



A greater number of segments (5) appear, however, to enter into its 

 composition. It may be pointed out that the direction of the fin is 

 not exactly similar to that of the pectoral fin, which is more normal 

 in this regard. One may suppose, however, that the absence of a 

 true pelvic arch has something to do with this. If one imagines a 

 partially aborted pectoral arch in the normal position, with the me- 

 tapterygials, etc., directed somewhat backwards, one would have an 

 intermediate stage between what obtains in the pectoral and pelvic 

 fins of the Teleosts. 



The Dorsal Fin Muscles.— The innervation of the erector es and 

 depressores of the dorsal fin is similar to that for the supracarinales, 

 i.e., the rain. lal. trigem. acts as a collector for the dorsal branches of 

 the spinal nerves, and gives off branches to the muscles. It would 

 seem, from the relations of these muscles, and also from their inner- 

 vation, that they are serially homologous with the supracarinales. 

 Dohrn's views 1 on the subject of the impaired fins receives confirma- 

 tion from the paired nature of the muscles, and still more from the 

 fact that a blood-vessel passes horizontally along through the base of 

 each ray, the ray splitting readily upwards from this channel, point- 

 ing to a coalescence of two parts, one on either side of the middle 

 line, in the formation of the fin. 



The Anal Fin Muscles. — With regard to the erectores and depres- 

 sores of this fin, the remarks made on those of the fin just described 

 apply equally well. They are really serially homologous with the 

 infracarinales. The lateral muscles of the anal fin are, however, of 

 an entirely different nature. Their innervation is from a superficial 

 plexus similar to that supplying the erectores and depressores. The 

 muscles lie completely outside the fascia covering the lateral muscles 

 of the trunk, and the plexus which supplies them is peculiar in being 

 in a similar manner superficial and formed from a plexus. The pro- 

 bability is that the muscles are dermal in their nature, and that the 

 plexus is a secondary one, produced from the deeper plexus already 

 present as the muscles gradually developed from the dermal tissue. 



The Caudal Fin Muscles. — These are nearly all modified portions 

 of the lateral musculature of the trunk. The intrinsic muscles are 

 not, however, but must probably be referred to the class of dermal 

 muscles. The innervation of the dorsal portions of the fin and of the 

 anterior continuation of that dorsal portion is interesting in showing 

 the relations of these parts to the dorsal and adipose fins. 



1 Dohrii. — Loo. cit. 



