1907.] NATURAL SCIENCES OF PHILADELPHIA. 103 



3. Nervous System. 



(a) Central Nervous System. — The central nervous system com- 

 prises a brain, a pair of circumocsophagcal commissures, and, con- 

 tinuous with the latter, a pair of longitudinal nerve trunks, provided 

 with ganglia and transverse commissures. 



The brain (figs. 1. 2, 19. 20 and 23), which is situated in the anterior 

 portion of the head, is made uj) of two distinct portions, an internal 

 portion composed of interwoven nerve fibres, the neuropil (np.), and 

 an external portion formed of several layers of ganglion cells (gr.c), 

 the outermost of which are in direct contact with the hypodermis. 

 The neuropil, which in sections may always be readily distinguished by 

 its yellowish tinge, as well as by its histological structure, has the 

 general shape of a plano-convex lens, with the convex surface directed 

 forward. It is, as may be seen in either horizontal or transverse sec- 

 tions, divided into three subequal lobes; one dorsal and median, and 

 two lateral. The latter probably correspond to the "lobioptici" of 

 iSchimkewitsch (1895). Near its centre it is, in addition, pierced by 

 a cleftlike aperture (fig. 2), Both the lateral incisions and the median 

 aperture are traversed by the anterior portions of the longitudinal 

 muscle fibres {l.jn.f.) described in the preceding section. The general 

 course of the nerve fibres which make up the neuropil is not immedi- 

 ately apparent ; the first impression received is that of a closely matted 

 and tangled mass. A careful inspection, however, shows that the 

 fibres composing the three lobes for the most part run parallel to their 

 surfaces, while those of the lower half of the fibrous mass, below the 

 median aperture, run from side to side. The latter constitute the 

 transverse commissure of the brain (figs. 2 and 20, br.com.). 



The neuropil mass is covered on its anterior, dorsal and lateral sur- 

 faces by several layers of ganglion cells ; its posterior surface is bounded 

 by the cavity of the head, while its lower portion is separated from 

 the ventral hypodermis by a narrow cleft (fig. 2). The ganglion cells 

 are imperfectly divided into four groups, giving to the brain as a whole 

 a lobed outline (figs. 1, 2, 19, 20 and 23). Three of these groups 

 correspond to the median and lateral lobes of the neuropil, and are due 

 to the same circumstance, namely, the arrangement of the anterior 

 portions of the longitudinal muscle fibres. The fourth group covers 

 the anterior surface of the brain, and is somewhat irregular in outline 

 (figs. 19 and 23). Of these four groups the dorso-median is note- 

 worthy in that it is situated comparatively remote from the rest of the 

 brain, and is connected with the neuropil by a distinct bridge of nerv- 

 ous fibres (fig. 23, ??../.). In fig. 23 a fifth group of ganglion cells is 



