Anatomy and Histology of Nemer tines. 405 
possess only one pair of neurochord- cells, which belong to 
the brain, and only a single pair of unbranched neurochords, 
which run through the brain and the lateral cords. 
The entire mass of the central nervous system is enveloped 
in a neurilemma. The fibrillar central substance of the 
lateral cords in all cases, and throughout Group II. that of 
the brain as well, is also enclosed in an inner neurilemma 
and sharply marked off from the coat of ganglion-cells. 
The peripheral nervous system is represented by nerves 
and nerve-sheaths. Nerves supply the cephalic extremity, 
the eyes, and the lateral pits. A pair of nerves, which arise 
from the ventral ganglion, runs back to the oesophagus ; a 
precisely analogous pair, springing from the ventral com- 
missure, supplies the proboscis in Groups I. and II. In 
Group III. the proboscis is innervated by means of numerous 
stems, arising from the brain. In some species of the first 
group, and in all those of the third, the lateral organs are 
united by nerves to the dorsal ganglia. In all the groups 
the lateral cords give off nerves, which are arranged meta- 
merically in Groups II. and III. In Garinina the nerve- 
sheath assumes an epithelial position, in accordance with the 
situation of the lateral cords ; in the other genera of this 
group the sheath is subepithelial. In Group II. the nerve- 
sheath is generally situated outside the circular muscle-layer, 
but it may occur within it, as in Langia and Gerebratulus. 
In Group III. nerve-sheaths are not found. The nerve- 
sheaths are characterized by the presence of a median dorsal 
nerve, which runs through them in the longitudinal axis of 
the body. This nerve also persists in Group III., only in 
this case it maintains an intermuscular position, above the 
circular muscle-layer. A second and smaller nerve of this 
kind, lying within the circular muscle-layer, is characteristic 
of the first two groups only. 
In close connexion with the nervous system come the 
sense-organs — the subepithelial eyes (the pigment-cups of 
w r hich are directed outwards), the lateral organs, the accessory 
lateral grooves lined with columnar epithelium ( Drepano - 
phones), and the terminal cephalic grooves ( Gerebratulus ). 
The lateral organs are placed in the same position as the 
brain, and in a portion of the genera belonging to the first 
group and in all those of the second and third they fuse with 
the dorsal ganglion, behind which they always lie. In the 
Enopla they occupy an independent position, being connected 
with the upper ganglion by nerves only, and generally lying 
to the side of it, though they may occupy a position in front 
of it, towards the cephalic extremity. As special formations 
