678 PROCEEDINGS OF THE ACADEMY OF [DeC. ^ 



IV, shows that what he regards as cross striatious are evidently 

 merely swollen, i.e., contracted areas of the fibres. 



2. The Nervous System. — a. Anatomy. — Burger (1895) 

 divides the nervous system of the Nemerteans into a central and a 

 peripheral system. The former comprises the brain and lateral 

 nerve chords, which have a thick investing layer of ganglion cells; 

 the latter, all other nerves and nerve layers, provided with a thick 

 ganglion cell envelope. Montgomery (1897 b), p. 382, includes 

 in the central nervous system all parts provided with ganglion 

 cells, namely: " (1) the dorsal and ventral brain lobes and com- 

 missures; (2) the lateral nerve chords . . . ; (3) the paired 

 oesophageal nerves; (4) the longitudinal nerves of the probos- 

 cis . . . ; (5) the dorsal, unpaired, larger median nerve of the 

 body wall; and probably also (6) the lesser, unpaired median 

 nerve. ' ' 



The anatomy of the brain of ZijgeupoUa may be readily studied 

 in life from a specimen compressed beneath a cover glass, for the 

 head is so transparent that the parts are easily distinguished (see 

 figs. 1, 16). The brain lies about 4 mm. behind the tip of the 

 head, directly in front of the mouth, encircling the rhynchocool, 

 and consists of four lobes or ganglia of about equal size, two dorsal 

 and two ventral. A^iewed from the dorsal surface, the dorsal 

 lobes, fig. 10, D.L., are most prominent, and at their posterior 

 ends are situated the pear-shaped ce/ebral organs, C.Org., which 

 are about one-third the length of the dorsal lobes and about one- 

 half their width. The dorsal lobes are connected above the rhyn 

 choctel by a slender commissure, which is very difficult to see in 

 life, figs. 1, 16, D.Comm., but is easily made out from sections. 

 Its dorso-ventral measurements vary in different specimens from 

 .04-07 mm., probably owing to diftereut states of contraction, 

 or to the plane of the section. In the sections of one si^ecimen, 

 fig. 19, the dorsal commissure appeared to be composed of fibres 

 coming from both dorsal and ventral lobes, an unusual condition, 

 and one not previously described in Nemerteans, so far as I am 

 aware. The ventral lobes are united by a stout commissure, about 

 14 mm. measured dorso-ventrally, that in life may be plainly seen, 

 figs. 1, 16, shining through the rhynchocciel, which lies above it, 

 and extending backward posterior to the dorsal commissure. Meas- 

 ured from side to side, both commissures are short, owing to the 



