THE MYOLOGY OF POLYODON Mie 
Blood supply. The myomeres are supplied throughout by the 
segmental arteries. Some of the anterior are also supplied in 
part by the a. thoracico-dorsalis. They are drained by the 
segmental and lateral abdominal veins. 
Action. This is the muscle of locomotion. Its contractions 
bring the head and tail toward each other, bending the body 
laterally. Through the attachment of the myocommata the 
action is not alone on the tail but on all the segments back of 
the skull. The corresponding lateral muscle of the opposite side 
is its opponent. 
The great ventral muscle: figure 10 
Embryological studies on a number of lower vertebrates have 
shown that the ventral musculature arises from a series of buds 
that form one at the end of each segment of the lateral muscula- 
ture. Consequently the ventral muscle is segmented in the same 
manner as the lateral. In the adult Polsodon the fifth segment 
of the lateral muscle is directly in contact below with one of the 
segments of the ventral muscle; each succeeding segment back 
to about the twenty-fifth is similarly related to a corresponding 
segment of the ventral muscle. Anterior to the fifth segment 
of the lateral muscle the segments of the ventral part, owing 
perhaps to the intervention of the pectoral fin, are no longer in 
contact with those above. Some of the anterior part is extended 
forward as an element of the coraco-arcualis upon which there 
are transverse inscriptions, but these are sufficiently numerous to 
preclude the possibility of their representing remains of primitive 
myocommata—unless there are a number of otherwise aborted 
postoccipital segments. The ventral muscle does not reach the 
median line, being separated from its fellow of the opposite side 
by a wide, tough, linea alba. It tapers posteriorly and ends in 
front of the vent almost asa point. Anteriorly its medial super- 
ficial fibers are slightly oblique, being directed forward and inward. ° 
Maurer (712, p. 38) designates these fibers as the musculus obli- 
quus inferior. He further subdivides the fibers of the remain- 
ing ventral musculature into mm. obliquus superior and obliquus 
medius. The latter is composed of fibers parallel to the former 
but covered by the obliquus inferior. 
