440 G. H. PARKER AND E. G. TITUS 



on the directives; it is continued peripherally, and in the com- 

 plete mesenteries centrally, into thin sheets of longitudinal 

 fibers which cover the rest of the mesentery. The longitudinal 

 muscles are absent from the very small incomplete mesenteries. 



9. The transverse muscles of the mesenteries (figs. 1, 2, mu. t.) 

 are very thin sheets that cover the endocoel faces of the direc- 

 tives and the exocoel faces of the other larger mesenteries; that 

 is, they are on faces opposite to those on which the longitudinal 

 muscles are located (Hertwig, 79-80, pp. 527, 530). They 

 are not only very thin but they are very uniform layers of fibers, 

 whose direction is approximately transverse to the chief axis 

 of the animal. Commonly, however, they slope a little orally 

 as they take their course from the periphery toward the center 

 of the animal. They are better developed on the complete 

 mesenteries than on the incomplete ones, from the smaller of 

 which they may be entirely absent. 



10. The parietal muscles of the mesenteries consist of longi- 

 tudinal ridges on the exocoel and endocoel faces of almost all 

 mesenteries at their line of junction with the column wall. On 

 the larger mesenteries these muscles are small and inconspicuous 

 in com.parison with the other musculature of these organs, but 

 on the very small mesenteries they are almost if not quite the 

 only muscles present. We have not attempted to distinguish 

 here what Carlgren ('05) would probably call the true parietal 

 muscle, namely, that on the endocoel side of a non-directive 

 mesentery, from what may be the remains of the parietobasilar 

 muscle on the exocoel side of the same organ, for the reason that 

 the parietobasilar may be entirely absent in Metridium. The 

 muscles on the two sides of the mesenteries in Metridium are 

 so strikingly similar that it seems to us more probable that we 

 are dealing with a pair of parietals, the parietobasilar being 

 absent (Hertwig, 79-80, p. 527, Taf. 17, fig. 1; McMurrich, 

 '01, p. 8), than that this muscle is present in a reduced condition 

 (Carlgren, '93, p. 107). 



11. The circular muscle of the column is a cylindrical sheet 

 of fibers covering the entodermic face of the supporting lamella 

 of the column from its attachment to the pedal disc to the 



