NERILLA AN ARCHIANNE LID. 
403 
head, where they are especially numerous in the region of the 
nuchal organ. Here the granules are of a brownish tinge, 
and lend a colour to the skin. 
The nervous system consists of a brain and two longi- 
tudinal cords extending on either side of the ventral ciliated 
groove to the posterior end of the body (figs. 2, 20, 35, 36). 
The whole system adheres closely to the epidermis, being, in 
fact, so intimately connected with it that no clear line of 
demarcation can be made out except on certain, parts of 
the brain. While the nerve-fibres are chiefly grouped 
on the inner surface, the ganglion cells occupy the outer 
region. The brain is a bilobed mass lying on the ventral and 
lateral walls of the prostomium below the origin of the ten- 
tacles (fig. 35). From it arise two oesophageal connectives, 
which pass down on either side to join the ventral cords (fig. 
37). No ganglionic swellings or aggregations are to be seen 
on these cords ; but fine transverse fibres connect the two, 
and nerves pass out to the peripheral parts. Unfortunately 
the nervous system does not differentiate with methylene 
blue, so it is scarcely possible to follow it out in detail. 
Sensory hairs occur on the palps, tentacles, and cirri ; 
especially near the distal ends of the joints. 
Of the four eyes the first pair is the larger, and is directed 
outwards and forwards (fig. 2). This is determined from the 
position of the distinct lens lying in the deeply pigmented 
yellow-brown cup. The hinder, smaller pair point outwards 
and backwards. 
As observed by Miss Pereyaslawzewa, the eyes are verv 
brilliant ; against a dark background they shine like the eyes 
of a cat in the dusk. The nuchal organ is in the form of a 
deep crescentic groove, with thick edges, and strong cilia 
on the posterior face of the depression (fig. 2). It communi- 
cates by a deep groove with the mouth (fig. 3). It has 
already been mentioned that lateral ciliated patches occur on 
each segment, which may possibly act as sense-organs (fig. 1). 
