362 
PACIFIC SCIENCE, Vol. XXI, July 1967 
DISCUSSION 
If they are represented by the rather linear 
reduction in gill arch elements, relationships 
among the examined genera may be as dia- 
grammed in Figure 12. Thus, Conger would 
be the most primitive and Gymnothorax the 
most advanced. Gill arch muscles of Conger 
are not structurally far removed from those of 
Elops (Nelson, 1967) or those of other gen- 
eralized lower teleostean fishes (Vetter, 1878; 
Dietz, 1912, 1914, 1921; Greene and Greene, 
1913). On the other hand, the muscles of 
Uropterygius or Gymnothorax are far removed 
structurally from those of Conger and conse- 
quently appear to be advanced rather than 
primitive. The series of studied forms ranging 
from Conger to Gymnothorax shows a progres- 
sive series of muscle modifications, involving 
the loss of some muscles and the appearance 
of others (Tables 1 and 2). The series of mus- 
cle modifications in a general way parallels the 
linear reduction in gill arch elements of these 
forms. 
Particular modifications of gill arch muscles 
in eels seem correlated with particular modifi- 
cations of the gill arches themselves. Reduction 
of ventral musculature (obliqui and recti) 
parallels reduction in ventral arch elements 
Fig. 12. Diagram of possible relationships among 
some eels. 
(basi- and hypobranchials) . Appearance of 
protractors and enlargement of the retractors 
and their attachment to the vertebral column 
in muraenines parallel the enlargement of the 
fourth arch and the tooth plates it supports. 
The appearance of the subpharyngealis is 
not so easily correlated with any particular 
modification of the gill arch skeleton. It ap- 
pears, seemingly, in place of the obliqui and 
recti. However, nothing is known of its em- 
bryonic development and it may or may not 
represent modified obliqui or recti. Its position 
is distinctive, being internal to the skeletal 
elements rather than external as are the obliqui 
and recti. Probably the ventral musculature 
shifted from a relatively external to a relatively 
internal position with the reduction and loss 
of basibranchials. In any event, it assumed a 
sheetlike form, gradually encroaching upon the 
gill slits, which in the more advanced eels 
(e.g., the muraenids) are reduced to small 
round openings. 
Dorsal and ventral paired retractor muscles 
are present in all of the eels examined. In 
most forms they are only partly distinct sub- 
divisions of the inner longitudinal muscle layer 
of the anterior esophagus. In eels of the sub- 
family Muraeninae, they acquire an attachment 
to the vertebral column. 
The taxonomic significance of retractors in 
other groups of bony fishes has been dealt 
with by Dietz (1912, 1914, 1921) and Holst- 
voogd (I960, 1965). According to Nelson 
(1967), retractors probably have developed 
an attachment to the vertebral column 
independently in many evolutionary lineages 
of bony fishes. Probably in each lineage they 
are associated with and constitute part of an 
improvement or specialization in the feeding I 
mechanism. 
It is hardly to be doubted that the attach- 
ment to the vertebral column has been acquired 
independently among the eels. No other group 
having retractor muscles has both dorsal and 
ventral retractors attaching in common to the 
vertebral column. Indeed, except among eels, 
ventral retractors seem to be lacking. Thus, 
the attachment to the vertebral column of 
muraenines no doubt is another example of 
independent development. In this case they 
attach to the tooth plates of the pharyngeal 
