MUSEUM OF COMPARATIVE ZOÖLOGY. 29 
and perpendicular to the long axis of the branch. They arise from cells 
of the mesoderm, most of which in this region are filled with vacuoles, 
and often project into the colom, But in my opinion the muscle cells 
do not themselves arise from such vacuolated cells, for at even an earlier 
stage (corresponding to Figure 21, Plate III.) one can distinguish thick- 
ened patches of elongated cells in the mesoderm which are undoubtedly 
the young muscle cells; but they do not show the slightest traces of 
being vacuolated, and in fact are sharply distinguished from the adjacent 
cells by their uniformly granular appearance and their deeper coloration. 
3raem (’90, pp. 124, 125) has already stated that the parictal muscles 
arise in pairs, and come to traverse the cœlom, not remaining in the 
body wall. The truth of this statement I can confirm in the case of 
the parietal muscles first formed, which lie near the future septum. 
Plate V. Fig. 42 shows the origin of the muscle fibres on both sides 
of the branch. They have already migrated into the ewlom. As Braem 
plainly states, the component parts of this pair of muscles, developed 
from the mesoderm, migrate towards «ach other and finally fuse into 
one unpaired mass, as we see in Plate III. Figure 26. It is perfectly 
evident, in this case at least, that both ends of two muscles originating 
far apart migrate in some manner towards each other so that the cor- 
responding ends come to lie close together. Such a migration cannot 
be accounted for merely by growth of the body wall. The ends of the 
muscle fibres must move relatively to the body wall. 
When the muscles have reached their permanent positions in a 
diameter of the branch, we find their ends attached to the cuticula. 
As the muscle fibres stain deeply in hematoxylin, they can be distinctly 
traced through the vacuolated and poorly stained cells of the body wall 
(Plate IIL Fig. 26). Figure 29 shows a bit of the wall mechanically 
separated from the cuticula, the end of the muscle fibre remaining in 
place. Fine lines can be distinguished in the contractile, deeply stain- 
ing portion of the fibre. The surface by which attachment is effected 
appears very slightly erenulated on longitudinal sections of the muscle 
fibre. I could not distinguish any structural peculiarity on the part 
of tho cuticula to which the muscle was attached, — nothing to indicate 
how attachment is effected. 
Freese (88, pp. 15, 22, Fig. 11) has described a similar method of 
attachment of the muscles to the cuticula for Membranipora.! 
! My friend, Dr. G. H. Parker, tells me that a similar method of attachment of mus- 
cle fibres to the cuticula occurs in Crustacea. According to Tullberg (’82, pp. 27, 44, 
45), the adductor muscle fibres are in Mollusks attached to the cells of the ectoderm, 
The same condition as in Mollusks seems to exist in Annelids (Hisig, ’87, pp. 25, 86) 
