20 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
(1) The radial longitudinal muscles are disposed in live pairs, 
the members of each pair being separated from each other by 
the width of the radial water canal, to the wall of which they 
are both attached. 
There are no special retractor muscles in Caudina such as exist 
in the genus Molpadia, but the anterior ends of the radial muscles 
serve to draw in the aquapharyngeal bulb. Each pair has its 
insertion as a single muscle in the corrugated outer surface of the 
anterior part of a radial calcareous plate. Toward the anterior 
extremities of the radial muscles the interradial edo'es of each of 
O 
those composing a pair curve inward toward each other, until 
they come' together. Tlie adradial edges of both approach each 
other and unite immediately beneath the radial water canal, so that 
anteriorly there is in each radius a single longitudinal tubular 
muscle, which includes for a short distance a central cavitv, lvinff 
deeper than the radial water canal. 
In the posterior part of the body the longitudinal muscles of 
each pair run backward as semicylindrical trunks close to each other 
but not in contact, as they are, according to Ludwig, in Ankyro- 
derma musculus. They taper gradually and are finally inserted 
into the connective-tissue layer of the body-wall at the tip of the 
so-called tail. 
The supposed pair of smaller longitudinal muscles, described and 
figured by both Clark (’65) and Kingsley (’81) as lying more super¬ 
ficial than the longitudinal muscles just described, and immediately 
below the radial nerve, do not exist. 
(2) The transverse muscles are limited to the five broad inter¬ 
radial areas lying between the pairs of longitudinal muscles. The 
fibers are inserted into the connective tissue a little to one side of 
each radius, and are not sufficients numerous to form in each 
interradius a continuous sheet, except when the animal is shortened 
by the contraction of the longitudinal muscles. In the anterior part 
of the body, just behind the tentacles, the fibers become continuous 
across each radius, thus forming an uninterrupted circular muscle, 
which serves, after the tentacles have been retracted, to constrict 
the head region. 
The parallel fibers composing the longitudinal muscle bands can 
easily be separated from one another by maceration in 20 per cent 
nitric acid. The fibers (Plate 2 , figs. 11, 12), are spindle-shaped 
tapering to a single point at either end. Each has an oval nucleus 
