NERVOUS SYSTEM 467 



The fore-part of the ring consists of the fused optic and protocerebral lobes. Between 

 the tips of these lobes there arise five nerves (Fig. 9), three running to the three divisions 

 of the nauplius eye and two arising more ventrally and passing to the frontal organ 

 (Fig. 2). Laterally, the optic lobes give rise to the short eye stalks of the paired eyes. 



On the ventral side of the fore-part of the nerve ring there are two swellings from 

 which the antennulary ner\'es arise. This marks the deuterocerebral part of the brain, 

 and in the hinder part of this region there is a small median aperture through which 

 passes the frontal apodeme and a portion of the sympathetic system. This aperture is 

 probably of little morphological significance as no fibres pass behind it. 



The deuterocerebral lobes pass postero-laterally into the tritocerebral region. There 

 is no marked waist between these regions, so that Hanstrom (1924, p. 32) is not strictly 

 correct when he states that, apart from the Phyllopoda, the Ostracods are the only 

 Crustacea which show a distinct separation between these two parts of the brain. The 

 tritocerebral lobes extend backwards and meet behind the oesophagus, thus forming 

 the anterior part of the hinder half of the nerve ring. The division between the trito- 

 cerebral commissure and the more posterior nervous mass is not marked by a gap, 

 but by a pair of muscles which pass through the nervous system. These have already 

 been mentioned (p. 453) in connection with the blood system, and are shown in Fig. 4 A. 



The nerves arising from the tritocerebral lobes are, anteriorly, the antennal nerves 

 which have double roots and, postero-dorsally, a pair of massive nerves which pass 

 outwards to the attachment of the adductor muscle where they enter the carapace. In 

 parasagittal sections they can be traced spreading out towards the margins of the valves. 

 I have called them the anterior valve nerves. Immediately below the latter a small 

 nerve emerges which runs to the posterior muscles of the base of the antenna. 



On the ventral side of the hinder half of the nerve ring there are two swellings, from 

 which arise the mandibvdar nerves and the roots of the sympathetic system (Fig. 10 A, B). 

 These are on the same level as the muscles which mark the hinder limit of the trito- 

 cerebral commissure. Just in front of them there are two small laterally projecting 

 lobes. They are probably of little significance, as no nerves arise from them and they 

 contain no fibre tracts. 



The postero-lateral corners of the nerve ring are marked by two prominent lobes — 

 the posterior lobes — which curve round the anterior hypostomal apodemes. They have 

 been figured by Miiller (1894, Plate 40, fig. 10). These lobes, together with the small 

 lobes at the sides of the mandibular swellings, do not represent any special division 

 of the nervous system, and probably arose mechanically as a result of the extreme con- 

 densation of the anterior part of the ventral chain. 



Immediately above the mandibular swellings, and near the dorsal surface of the nerve 

 ring, are the roots of two massive nerves which extend laterally and innervate the adductor 

 muscle. These nerves correspond to Turner's "thoracic nerve" (1896, p. 31) and to 

 Klie's "shell nerve" (1926, p. 23). 



From the dorsal tip of the posterior lobe a nerve runs obliquely forwards and enters 

 the shell at the adductor muscle attachment. It spreads out to the margins of the valves. 



