314 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



pose that originally it received a portion of the muscle, and that the 

 arrangement now seen in Gyprinus, Barbus, and Perca, is the older 

 one, that of Esox and Amiurus being the later modification. 



2. Adductor Tentaculi. — (PL III., Fig. 1, and 2, AT.) 



On cutting through the insertion of the add. mand. and reflecting 

 it, a muscle is exposed which is apparently characteristic of the 

 Siluroids. It arises from the outer surface of the metapterygdid, its 

 upper portion being covered by the lev. arcHs palatini. It runs for- 

 ward beneath the add. mand., forming the inferior boundary of the 

 orbit and being crossed by the fifth nerve. Anteriorly it becomes 

 tendinous, the tendon near its insertion dividing into two slips, be- 

 tween which the nerve supplying the tentacle passes. One of these 

 slips is inserted into the upper, the other into the lower border of the 

 base of the maxilla, which encloses the proximal portion of the 

 tentacle. 



Innervation. — It is supplied by a branch of the same nerve that 

 supplies the deeper portions of the add. mand. 



Action. — It draws the tentacle backwards towards the middle line, 

 opposing the anterior portion of the add. arcu$ palatini. 



The position and innervation of this muscle leads to the conclu- 

 sion that it is a part of the add. mand. which has been separated off 

 for a particular purpose. It does not, however, compare with any 

 of the three parts of that muscle in Barbus or Perca, nor even with 

 the fourth part, which is sometimes present, as in Cyprinus, since 

 this is formed by a division of the superficial portion. Since the 

 osseous support of the long tentacle is the maxilla, this muscle 

 bears a certain amount of analogy to the superficial portion of the 

 add. mand., but it cannot be its homologue. The relation of the 

 maxilla to the tentacle was probably secondary, and since the power 

 of moving the tentacle would always have been an advantage it is 

 probable that originally the muscle was inserted into the tentacular 

 cartilage, its insertion into the maxilla only occurring after that bone 

 had commenced to be a support and had enclosed the base of the 

 tentacle. There are two theories which will account for the presence of 

 this muscle. (1) It may be a new structure evolved for a particular 

 purpose, or (2), it may be the representative of a muscle present in 

 ancestral forms but which has disappeared in all the Teleostei 

 hitherto examined. If the latter is the correct explanation, one 



