COE: TERRESTRIAL NEMERTEAN OF BERMUDA. 557 
mainly of a thick central mass of endodermic cells, surrounding the 
primitive digestive cavity, and a thin outer layer of ectodermic cells, 
which are doubtless covered with delicate cilia. 
The body of the embryo developing from the gastrula becomes 
somewhat elongated, the ectodermic layer thickens, and the blasto¬ 
pore together, apparently, with the whole lumen of the primitive 
alimentary canal becomes obliterated. 
Nervous system .— In a short time the central nervous system 
(brain) develops from invaginations of the ectoderm, with which it 
remains in close contact (pi. 24, fig. 10), as is the adult condition 
in the Paleonemertea, until a late period in the development, when 
muscular and connective fibers grow in to separate it from the over- 
lying integument. The lateral nerves to all appearances represent 
outgrowths from the ventral brain lobes. They gradually extend 
backward from the ventral ganglia toward the posterior end of the 
body, remaining for a long time either in or immediately beneath 
the integument. 
At a somewhat later stage, when the outline of the body has 
become considerably elongated, the brain lobes become very large as 
compared with the size of the body (pi. 24, figs. 10, 11) and the 
ventral commissure very conspicuous. The lateral nerves have a 
small central fibrous core and a comparatively thick layer of nerve 
cells with distinct nuclei (pi. 24, figs. 12, 16). 
Cerebral sense organs .— These sense organs are very conspicu¬ 
ous at the time of their formation, and are comparatively much 
larger in the young worm than in the adult. They arise as a pair 
of invaginations from the ectoderm of the lateral surface of the 
anterior portion of the body somewhat in front of the brain. Each 
sense organ is represented in the late embryo (pi. 24, fig. 12) by a 
conspicuous tube lined throughout with ciliated columnar epithelium. 
This tube passes through the integument and abuts against the 
antero-lateral border of the ventral brain lobe. It is broadest in 
the integument and is conspicuously constricted at the point where 
it passes through the basement membrane. Its inner portion ends 
blindly in a small chamber lined with ciliated columnar cells, which 
are similar to those of the tube and are imbedded in a mass of nerve 
cells more or less continuous with those w T hich form the ganglion 
cell layers of the brain lobe. From the posterior end of this cham¬ 
ber a narrow canal soon grows posteriorly for a short distance along 
