186 CHAS. B. WILSON. 



exactly with the ring of muscle fibres and cells which lie at 

 the base of the ciliated band in the trochophore larvae. 



The cells are small, and look like micromeseucytes, except 

 that they are mostly unipolar, the single pole giving off a long 

 fibrous process. The row of cells can be followed from the 

 lappets around the border of the body, but the muscle band 

 blends with the circumoral muscle, and loses its indi- 

 viduality. 



This locomotor muscle lies in exactly the position assigned 

 by Salensky to the " nerve-ring/' fig. 79 is almost identical 

 with fig. 4 of his description. But we have called them 

 muscle for the following reasons. Salensky states that " the 

 nerve-ring passes along the cilia row, and thus becomes the 

 homologue of the nerve-ring in the annelid larvae described 

 by Kleineuberg." But he fails to note that Kleinenberg also 

 describes a ring of muscle cells at the base of the ciliated cells, 

 which is used by the larva in locomotion (29). 



If either of these sets of locomotor apparatus are to be 

 developed in so primitive a larva as the pilidium at the ex- 

 pense of the other set, the presumption is strongly in favour 

 of the muscle. We would scarcely expect to find the nerve 

 well developed, as Salensky claims, and no trace whatever of 

 the muscle. 



Again, borax carmine, orange G, and hsematoxylin stain 

 these fibres exactly the same as the other pilidium muscles, 

 and no difference is perceptible in their structure or texture. 



Another proof is that the cells whence the fibres originate 

 are exactly like those from which the radial fibres arise. 



It has never been doubted that the latter are anything but 

 muscle cells ; they, too, are unipolar or multipolar, as Salensky 

 claims for the " nerve-cells," and his argument from form 

 would apply equally well to both. Finally, this strand is 

 continuous with the circumoral muscle (fig. 77), and if one 

 is nerve, the other must be also ; but if we concede that the 

 circumoral ring is muscle, then the strand must also be 

 muscle. 



These facts, no one of which, perhaps, is sufficient alone, 



