MUSEUM OF COMPARATIVE ZOOLOGY. 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 coelom. 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. 



Braem ('90, pp. 124, 125) has already stated that the parietal muscles 

 arise in pairs, and come to traverse the ccelom, 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. Pig. 42 shows the origin of the muscle fibres on both sides 

 of the bi'anch. They have already migrated into the coelom. As Braem 

 plainly states, the component parts of this pair of muscles, developed 

 from the mesoderm, migrate towards each 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 hoematoxylin, they can be distinctly 

 traced through the vacuolated and poorly stained cells of the body wall 

 (Plate III. 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 eflfected 

 appears very slightly crenulated on longitudinal sections of the muscle 

 fibre. I could not distinguish any structural peculiarity on the part 

 of the 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.^ 



1 My friend, Dr. G. H. Parker, tells me that a similar method ot attachment of mus- 

 cle fibres to the cuticula occurs in Crustacea. According to TuUberg ('82, pp. 27,44, 

 45), the adductor muscle fibres are in MoUusks attached to the cells of the ectoderm. 

 The same condition as in Mollusks seems to exist in Annelids (Eisig, '87, pp. 25, 36). 



