134 Cc. H. DANFORTH 
Their tendency to remain distinct may indicate their primitively 
separate nature. 
VI. MUSCLES OF THE TRUNK 
Lateral trunk muscle 
The musculature of the trunk is in two separate parts, the 
lateral and the ventral. The former, which can very well be 
considered as a single muscle, arises from the occipital part of 
the skull and from the strong fascia back of the branchial region 
and is inserted in the caudal fin, both above and below the level 
of the notochord. It is crossed by about fifty-eight tendinous 
septa, the myocommata, which divide it into its myomeres and 
attach it firmly to each segment of the axial skeleton. The 
fibers, which run in a longitudinal direction, arise from one myo- 
comma and are inserted on the next posterior. The muscles of 
the two sides are in contact with each other dorsally throughout 
the whole length, except where separated by the dorsal and caudal 
fins, and ventrally between the posterior end of the anal fin and 
anterior margin of the caudal. They approach but do not actu- 
ally meet along the line between the pelvic fins and vent. ‘Toward 
the insertion, above the notochord and within the caudal fin, the 
dorsal (epaxial) part of the muscle becomes decidedly tendinous. 
This is also true, but to a much less degree of the hypaxial part. 
* The several longitudinal body muscles recognized in teleosts 
are not differentiated in Polyodon, although homologous regions 
may be more or less clearly indicated by the foldings of the 
myocommata and muscle segments. The so-called dermal mus- 
culature is clearly evident both in dissections and in transverse 
section, where it can be distinguished by the smaller fibers which 
are more loosely arranged. It is, however, segmented and each 
segment is directly continuous with the subjacent myomere. 
Since the septa do not cross the muscle in a strictly transverse 
plane but proceed from axis to periphery in a zigzag fashion, the 
separate myomeres are somewhat complicated in form. Fig. 9 
represents a dissection. of the tenth one. Roughly speaking, it 
is in the form of a cone, the medial half of which has been cut 
away. Its apex lies just above the notochord and points forward. 
