GENERAL ACCOUNT OF NEMERTEA. 199 
as a schizoccele, z.e. a space formed by the cleavage of the mesoderm 
into two layers (Pilddiam-larve). In the adult only the blood spaces 
and the cavity of the proboscis sheath are coelomic. The nervous 
system consists of 2 brain generally four-lobed,—the two lobes of each 
side being closely united and connected with those on the other side by 
a commissure above and by another below the proboscis cavity. From 
the lower lobes two longitudinal nerve-stems run along the sides, and 
are sometimes united posteriorly above the anus (Fig. 104, 2.7.). In 
some forms there is in addition a dorso-median nerve, and sometimes a 
ventro-median nerve. 
On each side of the head there is a ciliated pit communicating with 
the exterior through an open slit or groove, and communicating inter- 
nally either with the brain itself or with adjacent nervous tissue. In 
those cases in which the development has been studied, these so-called 
lateral organs arise from 
epiblastic insinkings and 
cesophageal outgrowths, In 
the most primitive genus, 
Carinella, they are absent, 
except in one species. It 
has been suggested that they 
conduce to the respiration of 
the brain, which is rich in 
hemoglobin, and they have 
even been compared with 
gill-slits. In some forms the 
groove through which they 
open to the exterior is rhyth- 
mically contractile. It has 
also been suggested that they 
are sensory. Apart from ‘ : 
these organs, Nenfertines are, din, Doral neve ic mobs cavity 
Wery. sensitive, and an many. filing, cadssless BUM, dorso- ventral by 
this is associated with a super- — diagonal muscles; Zv., lateral blood vessel. 
ficial nerve plexus. Tactile 
papillae and patches are often present; eyes and eye spots are 
general ; and in some there are otocyst-sacs. Apart from the cephalic 
slits, the head also bears sensory pits and grooves and terminal 
sensory spots. In some there is a pair of lateral sense organs 
in the (anterior) nephridial region. The mouth is ventral, and leads 
into a plaited glandular fore-gut or cesophagus, which is followed by a 
straight, ciliated mid-gut (stomach and intestine), usually with regularly 
arranged lateral caeca. Between the cxeca run transverse muscle parti- 
tions. The anus is in most cases terminal. In a cavity along the dorsal 
median line there lies the remarkable proboscis. It is protruded and 
retracted through an opening above, or, in a few cases, within the 
_mouth. It arises as an invagination from infront, and is a muscular, ° 
very richly innervated tube lined with glandular epithelium, sometimes 
protruded with such force that it separates from the body, and then 
often retains its vitality for a long time, as if it were itself a worm. It 
has been compared in its retracted state to a glove-finger drawn in by 
Fic. 105.—Transverse section of a simple 
Nemertean (Carznella).—After Biirger. 
