RESEARCHES ON THE NERVOUS SYSTEM OF NEMERTINES. 279 
(to which the majority of hitherto known Nemerteans 
belong) as entirely homologous with the simple opening 
described for Polia, ValencAnia, ^c. ; this opening having 
here been widened out and deepened for purposes in direct 
connection with the strongly augmented amount of haemo- 
globin contained in the nerve-cells, and with the scarcity 
of oxygen in those places where the species of this suborder 
are found (deep in the mud, under stones, or in decaying 
matters on the sea bottom). The respiratory function of 
these slits, which was tested by the author in different 
physiological experiments, described in the original memoir, 
can be exercised, 1st, at the surface of the two pairs of 
anterior lobes, where these are externally visible at the 
bottom of the slits, and 2nd, in a more complete way inside 
the third (posterior) cerebral lobe (which has here become 
somewhat more separated from the anterior than was the 
case in Polia), where the ciliated duct with its continual 
current of^fresh seawater is immediately surrounded by the 
hsemoglobinous nerve-cells (see Pl.XXIlI,fig. 5). In some of 
these species the ciliated canal divides into two, one penetrat- 
ing among the nerve-cells, the other apparently remaining 
more externally situated in relation to the ganglionic lobe. 
Both have blind terminations, and never show any trace *of 
a special sensory epithelial layer. The accessory mass of 
large cells (pale in fig. 5) is somewhat more conspicuous but 
occupies the same place as in Polia. 
In the Hoplonemertini differentiation has gone on along 
another tract. The transverse furrow has not been modified 
into a longitudinal slit, but in certain genera {Drepano- 
phorus, Amphiporus) it is even provided with short longi- 
tudinal grooves perpendicular to it, exactly as it is found 
in Polia, whereas in other genera these secondary short 
grooves are absent. Internally, another important change 
is the entire separation of the posterior cerebral lobe from 
the anterior dorsal one, with which it is still so closely 
united in Polia, and with which it is here only connected 
by one or more nervous commissures. With this separation 
a greater independence in its position has been gained it 
may either be situated behind, on a level with, or before 
the rest of the brain. Here, as in Polia and the Sciiizone- 
MERTiNi, there is a grouj) of cells of quite different appear- 
ance coalesced with the ganglion cells, which latter, however, 
form tlie bulk of the posterior lobe ; but here a ciliated 
space or canal remains free in the midst of this accessory 
mass of cells. The primary ciliated canal ])cnetratos from 
the exterior into the midst of the true ganglion cells ; into 
VOL. XX. NEW SER. T 
