1901.] NATUKAL SCIENCES OF PHILADELPHIA. 681 



nervous layer of fibrous substance (" plexus " of Ilubrecht, " Ner- 

 venschicht " of Burger), situated outside the circular muscle. It 

 is especially strong in the (esophageal part of the body, and in the 

 inner circular muscle region, fig, 23, n.p. Posteriorly the layer 

 is very thin, or may be entirely absent. 



In one specimen a peculiar condition of the lateral chords was 

 observed, Plate XLIV, fig. 62, L.N. A part of the fibrous sub- 

 stance extends toward the centre of the section in between the fibres 

 of the circular muscle, CM. : the fibres in the same radial line as 

 the nerve are bent out of their course, and in the space thus 

 formed is the apparent branch of the nerve, running inward. This 

 condition was not confined to a few sections, but extended over 

 several slides, a distance of at least 1.5 mm. It at first seemed 

 as if a series of nerves was being given oflf from the inner lateral 

 face of the nerve chord, instead of from tlie dorsal and ventral 

 sides, the usual method in Nemerteans (Burger), Several other 

 specimens were examined, but this peculiarity was not found in 

 them, and I am unable to account for it. 



The proboscis is innervated by two slender branches that arise 

 from the dorsal surface of the ventral commissure and immedi- 

 ately run dorsally into the proboscis, which is attached at this point 

 to the body wall. The two proboscis nerves, Plate XLII, fig. 35, 

 P.N., are distinct in the anterior part of the proboscis, but in the 

 " middle region," as will be described in that section, they become 

 a continuous nervous layer, fig. 40, n.p., separating again near 

 the end of the proboscis into, two distinct nerves, fig. 41, P.N. 



h. Histology of Nervous System. — The brain lobes, as described by 

 Burger (1895) and Montgomery (1897 b), consist of (1) the 

 fibrous core, (2) the inner neurilemma, (3) the ganglion cell layer, 

 and (4) the outer neurilemma. 



There is little to be said in regard to (1) and (2), as these 

 structures in Zygeupolia conform to the usual Nemertean type. 



In the ganglion cell layer of the brain occur the three types of 

 nerve cells, the small, the medium-sized and the large, described 

 by Burger and Montgomery and denoted as I, II and III. Mont- 

 gomery describes the cells of the first type, p. 385, as " densely 

 massed together and of a shortened pyriform shape. The nucleus 

 is very large in proportion to the cell body, in fact, nearly filling 

 it . . . ." This de.-cnption may also be applied to the cells of 



