NEMERTEA 121 
smaller, lies dorsal to the proboscis sheath, the other between 
the proboscis sheath and the oesophagus. It will thus be 
seen that the proboscis is surrounded by a nerve ring, a re- 
lationship to the nervous system which is usually confined to 
the alimentary canal of Invertebrates. 
The dorsal and ventral ganglion are separated by a deep 
eroove. The dorsal half gives off nerves to the eyes and 
fore part of the head. The ventral half is continuous with 
the main lateral nerve trunks. These two trunks run back on 
each side of the body, embedded in the layer of longitudinal 
muscle fibres. In Schizonemertea there is a third lobe borne 
on the dorsal aspect of the brain; and in Hoplonemertea this 
lobe may be distinct and connected with the brain only by 
a nerve. In some species it is hollow, and its walls ciliated. 
In the last-named subdivision the longitudinal nerves give 
off numerous segmentally-arranged nerves, but in the Palaeo- 
and Schizo-nemertines these are replaced by a delicate plexus 
which lies between the external longitudinal and the circular 
muscles (Fig. 80). The main trunks may unite above the 
anus in the Hoplonemertines, as in Peripatus and Chaeto- 
derma. A median nerve runs back from the supra-proboscidian 
commissure and supplies the proboscis sheath and proboscis. 
The sense organs in Tetrastemma consist of four eyes 
which seem to be little more than pigment spots devoid of 
lens or other accessories. 
A ciliated groove exists on each side of the head; each of 
these leads into an oval sac which comes into close relationship 
with the cerebral ganglia. These are the lateral organs, and 
their nature is the cause of much discussion. They appear to 
arise in the Schizo- and Hoplo-nemertines partly from the 
epiblast of the skin and partly from the oesophagus. In the 
Schizo-nemertines, where the nervous system is coloured red 
with haemoglobin, they have been regarded as_ respiratory 
organs; but this does not explain their use in the other two 
subdivisions, and they have been variously regarded as sense 
pits and as excretory organs. The arrangement of their ex- 
ternal ciliated openings affords a useful basis for classification. 
Tetrastemma, like most members of the class NEMERTEA, 1S 
dioecious. The ovaries and testes are arranged along each 
