252 W. Ji. Coe — Develoi)ment of the 



between this one and the lower border of the pilidium. Both were 

 fully developed, and a distinct set of muscular fibers ran from ^ach 

 of them to the lappets. All such cases must, of course, be looked 

 upon as abnormalities. 



Salensky mentions (28) that some of the cells of the apical plate 

 are continued internally into slender processes which he considers to 

 be nerve-fibers. I was quite unable to distinguish any such nerve- 

 fibers from the numerous muscle-fibers which have their centers of 

 attachment among the cells of the plate. The relations of the cells 

 of the plate with these muscular fibers and the cells of which they 

 are outgrowths are shown in Fig. 6, PI. xxxv. As will be seen from 

 the figure the muscle-fibers do not come off directly from the plate- 

 cells themselves, but rather they are fastened into the intercellular 

 cementing substance. Whether Salensky mistook these fibers for 

 nerve-fibers it is impossible to decide. 



Both from its position and general appearance the apical plate of 

 the pilidium is naturally looked upon as homologous with that of the 

 trochophore-larva of Annelids. In both groups this plate usually 

 bears a tuft of long cilia or a single flagellum. From it muscles run 

 similarly to the digestive tract and to the sides of the body. This 

 organ in the trochophore, as is well known, represents the " Anlage " 

 of the superior esophagal ganglion of the adult. The sensory nature 

 of this plate in the pilidium has, for these reasons, been generally 

 conceded. On this account it is very disappointing to be unable to 

 find (by the use of methylene blue, or otherwise) any trace of differ- 

 entiated nervous structures in the young pilidia under consideration. 



The Lappets. 



At the end of the first day the ectoderm of the lower, lateral edges 

 of the embryo becomes extended to form the side-lobes, or lappets, 

 of the pilidium. These increase in size very slowly in M. cceca and 

 C. leidyi, and at the end of two weeks are, in comparison with those 

 of many other species, only slightly developed (Fig. 6, PI. xxxii). 

 It seems probable that this rudimentary condition of the lappets 

 remains thoughout the life of the pilidium, since they are here 

 decidedly longer than those described for the mature Pilidium auri- 

 culaticm of Europe. 



In C. leidyi the lappets are developed to a greater extent than in 

 31. cceca, as is seen from a comparison of Figs. 5 and 6, PI. xxxii.* 



* It should be noted that the magnification of the two figures in not the same. 

 The pilidia of both species are of nearly equal size. 



