THE MYOLOGY OF AMIURUS CATUS. 341 



muscle do not arise from the succeeding axial ray but may be traced 

 across it and several others to the fascia near the axial line, so that, 

 viewed as a whole, their arrangement resembles that of a fan. The 

 central muscles above and below lie entirely on the axial surface of 

 the ray to which they are attached, and, since there is no median 

 impaired ray, their fibres arise from the fascia between them and 

 partly also from the fascia of the superficial muscle. These intrinsic 

 muscles approximate the rays, being aided by the upper and lower 

 portions of the deep musculature and opposing the upper and lower 

 portions of the superficial muscles. 



Innervation. — The intrinsic muscles are supplied from a plexus 

 formed by ventral branches of spinal nerves xxxiii.-xli. The 

 muscles above the spinal cord are supplied by branches from R. lat. 

 trigem., and from the small posterior R. dorsales spinales. 



On comparing the myological characters of the head of a Teleost 

 with those of a Selachian, the first point that strikes one is the ab- 

 sence in the former of the well-marked constrictors found superfici- 

 ally in the latter ; in other words, the direction of the muscle fibres 

 in the Teleosts appears to be more longitudinal than in the Selach- 

 ians, and therefore the myomeres more similar to those of the trunk. 

 It has been shown by Balfour and Gcette that the musculature of the 

 head develops in exactly the same manner as that of the trunk, i.e., 

 from the primitive vertebrae, and is, therefore, segmental in its ori- 

 gin, a myomere lying between the arches of each pair of vertebrae of 

 which the head is composed. In Amphioxus there is no differentia- 

 tion of the myomeres, the musculature from the tail to the head con- 

 sisting of a series of similar myomeres separated by similar myocom- 

 mata 1 , and therefore represents more closely the original type than 

 does the arrangement in either the Selachians or the Teleosts. Ac- 

 cordingly, the Teleosts would at first seem to present a more primi- 

 tive type than do the Selachians, but a closer investigation shows 

 this to be a mistake. 



When one takes into consideration the presence of an osseous, and 

 therefore niore or less immovable, cranial skeleton in the Teleosts, 

 the absence of the constrictors is easily understood. But even then 

 one would suppose that in the more movable parts the constrictors 



1 The ventral musculature of Amphioxus would interfere with this generalization were it not 

 'that it must be considered as belonging to a different category from the trunk musculature. 



