276 
DR. H. W. HUBRECHT. 
stance prevails over the cellular in this genus, and is some- 
what loosely arranged. It is not surrounded by nerve-cells, 
as these form only an external coating to it, whereas in the 
higher-developed genera the cellular substance (in the brain 
at least, not always in the lateral trunks) does surround it 
on all sides (PI. XXIII, figs. 4, 5, and 6). This cellular por- 
tion in Carmella is also of a less compact nature than in 
those of more differentiated genera, and is ’everywhere in 
direct contact with the epidermoidal tissue. This may be 
regarded as the more primitive stage, in which the entire 
central nervous system, from the head of the animal down 
to the tail, has not yet become separated from the ectoderm 
through intervening muscular tissue. 
The ScHizoNEMERTiNi and the genera Polia and ValeU” 
ciniuj among the Paljeonemertini, represent a further stage 
of development, inasmuch as the central nervous apparatus 
is here situated exteriorly to the circular and interiorly to 
the outer longitudinal muscular layer, and so is everywhere 
surrounded by muscular tissue ; whereas in the large ma- 
jority of these genera the longitudinal trunks (‘^Nerven- 
markstamme,” as the author proposes to designate them) 
occupy a strictly lateral, opposite position (PI. XXIII, fig. 8) 
the genus Langia, with the curious longitudinal depression 
along the back, shows the well-marked tendency, already 
mentioned above, of an approxim action of these two parallel 
trunks on the dorsal side (PI. XXIII, fig. 9). In none of all 
these genera could a commissure uniting the lateral trunks 
in the posterior extremity of the animal be detected. They, 
simply terminate. This commissure makes its appearance in 
the Hoplonemertini, which are, moreover, characterised 
by the lateral trunks being situated wholly interiorly to 
the muscular body wall. So in this respect they form the 
opposite extreme to Carinella, 
Generally the position of the nerve-trunks is strictly 
lateral (PI. XXIII, fig. 10), also in this suborder ; in certain 
genera, however, as Drepanophorus (PI. XXIII, fig. 11), 
the longitudinal trunks have approached each other on the 
ventral side, and would, indeed, if they were connected by 
commissures or eventually coalesced, form an oesophageal 
nerve-ring and a ventral cord corresponding to the Bauch- 
mark” of Annelids and Arthropods. But such commissures 
fail in this genus, as they do in all other Nemerteans, and 
even in Drepanophorus the longitudinal trunks bend up- 
wards in the extremity of the tail, and are in direct com- 
munication by a commissure above the digestive tract. This 
large amount of variability in the position of the central 
