THE MYOLOGY OF AMIURUS OATUS. :U3 



limited, may explain the want of relation of the myomere to it, but 

 still one would expect to find the muscles iu relation to parts situ- 

 ated near it, i.e., in front of the orbit. In the Selachians this is the 

 case ; the origin of the add. mand. is in these forms entirely in front 

 of the eye, and its action is essentially that of a constrictor. It seems 

 that there has been first of all a gradual passage backwards of the 

 origin of the add. mand., (and also of the other trigeminal muscles), 

 until in the Teleosts it has come to lie entirely behind the orbit, and 

 that secondarily, thei-e has been a downward growth of the muscle, 

 so that the fibres have extended on to the hyomandibular, &c, the 

 lowermost assuming a horizontal direction. The relations of the ori- 

 gin of the add. mand. in the Cyprinoids, Perca and Esox, are in sup- 

 port of this supposition. Vetter has pointed out that the add. mand. 

 of the Cyprinoids is very much specialized, that of Perca slightly less 

 so, and that of Esox, to which Amiurus is most comparable in this 

 matter, more primitive than either ; and we find that in Esox, the 

 most primitive form, the muscle arises in part from the cranial bones, 

 (viz., the pterotic and sphenotic), whereas in the others the origin has 

 passed lower down. 



Why there should have been this passage backwards of the muscle 

 to behind the orbit, it is rather difficult to say. Perhaps an explana- 

 tion may be found in the fact that the muscle acts in the Teleosts 

 more or less as a retractor of the mouth parts, justifying in this 

 respect Owen's designation of it as the retractor oris. If an upward 

 movement of the mandible were all that was required, the arrange- 

 ment which obtains in the Elasmobranchs would certainly be most 

 effective, whereas, if retraction were also required, such a backward 

 progression would be necessary. 



It may also be pointed out that since the muscle lies entirely 

 behind the eyeball, the size of that structure will necessarily assist 

 in determining the extent of the limitation of the origin to the 

 hyoid arch. In Amiurus where the eye is so very small, the origin 

 persists much further forward than in any of the other forms 

 examined, in all of which the eyeball is comparatively large. 



The adductor mandibular of the Teleosts has been derived from a 

 constrictor muscle ; its relations to the hyoid arch have been produced 

 by a necessity for its action as ajretractor oris ; and the extent of its 

 departure from its original position is partly determined, by the size 

 of the eyeball. 



