MKMOlUy OF TIIIC NATIONAL ACADKMY OF SCIKN0K3. 30:5 



AstLe kiiile passes throufjli tlie posterior region of tlie liiU'iiil-eye lobes it also cuts tliiriii;cli 

 the upper ami anterior rey;ioii of the luediau t^ye lobes (Figs. 4 ami 5). 



In tlio next section liguretl, Fig. (!, the knilc passed lieliind the lateral-eye lobes, involving 

 tlie niediau-eye lobes, ami also the anterior region of the cerebral loltes which send loiiules 

 into the lower i»artnr tlie brain. The section, like all the others, being a little obliipic, show the 

 white librous matter on the right side penetrating to the bottom of the brain. Fig. (!« is an 

 eidarged view of the central portion of the left median eye lobe, showing the large ganglion ct^lls, 

 which are seen to vary much in si/.e; and tiie small gaiiglionicor chromatic <'ells forming the semi- 

 circular sheet or plate, and imlosing tlm librous matter as well as the myeioiil or i)unctiired sub- 

 stance. The inner layers of aprotoplasmie or chromatic cells (small ganglion cells) are here seen 

 to send their tibrillu' in toward the center of the mass inclosed by the cells. 



These snmll ganglion cells contain usually more than one nucleolus — usually two or three — and 

 1 have counted in them from 1 to (> nucleoli. The, number of nucleoli in the large ganglion cells 

 varies from 1 to-t. but tliere are usually two. These cells are seen to be unipolar and to send their 

 nerve fibers directly across toward the group of cells in the opposite lobe, though no distinct com 

 missure is thus formed. 



In a few sections farther back the ganglion cells, both large and small, are arranged as in 

 PI. XI, Fig. 7, and IM. Xli, Fig. .S shows their arrangement and relations to tlie central and lateral 

 large ganglion cells in a section made still farther back, near the middle of the brain. 



In the section represented by PI. XII, Fig. 9, which passes through near the middle of the 

 brain, the two masses of large ganglion cells of the median-eye lobes have coalesced, while the 

 cerebral ganglia are now larger, and the central and cortical groups of large ganglion cells are 

 larger than in Fig. 8. In the third section succeeding large ganglion cells appear on the h;ft side. 

 In PI. XIII, Fig. 10, the cortical groups of large ganglion cells disappear, and those of the central 

 groups become more numerous. This section involves the origin of the cesophageal ring, and now 

 the posterior end of the brain becomes Hatter, wider, and not so deep. PL XIU, Fig. 12, repre- 

 sents the i)osteiior portion of the cerebral lobe, with the thick cerebral commissures. In the 

 second section still farther back the ganglion of the first pair of api)endages, PI. XIII, Fig. 13, 

 is intersected, and in PI. XIV, Fig. 17, the knife passes through the oesophageal ring just behind 

 the ganglia of the first pair of appendages. 



.In the transverse sections of the brain of a small Liraulus, about 2 inches long exclusive of the 

 caudal spine, the details brought out, in addition to those already pointed out, are the very distinct 

 basal portions of the white fibrous portion of the cerebral lobes. These are seen in PI. XIV, Fig. 

 18, to send an irregular l)raneh on each side of the median line of the brain (/») up to the top of the 

 brain, each branch sending three irregular lobes out to the side of tlie brain, besides a number of 

 smaller ones which are seen to be cut across, and enveloped by the thick sheets or cylinders of 

 small ganglionic or chi-omatic cells. (These branches of the cerebral lobes are what Viallanes re- 

 gards as the stalk of the mushroom or pedunculated bodies). In Fig. 18 the median fissure or space 

 (m) between the sausage-like groups of chromatic cells is seen more distinctly than in the fully grown 

 individuals. In this section also the central groui)s of large ganglion cells {ch/c) are seen above 

 the cerebral commissiu-e {c. com), and are also arranged one group on each side of the median hue 

 of the brain. 



PI. XV, Fig. 20, represents a section througli the Iiindermost jjortion of the brain, the section 

 also passing through the ganglia of the first pair of apiM-ndages. It will also be seen here, as in 

 PI. VI, that the pair of ganglia of the first pair of appendages are quite distinct from the brain, 

 the nerves to these appendages {1st app. n.) arising from the mass of large ganglion cells forming 

 a part of the cesophageal ring. 



VIII. Kesults comi'aueu with tuose of other observers. 



As may be seen from the preceding descriptions and by reference 1x) the plates, the results 

 of our examination of the brain of the adult Limnlus, made with every possil)le care and ))ains, and 

 basi'd OM many hundred sections, cut not only in a liori/ontal. but also longitudinal and trans\eisc 

 direction, are the following: The brain of this merostome is composed of but three pairs of lobes, 



