ON PHYMOSOMA VABIANS. 17 



at the dorsal lateral angle of the brainy just dorsal to where 

 the circumoesophageal nerve-commissure leaves the brain ; the 

 lumen of the inner limb dilates into a vesicular swelling in the 

 substance of the brain (fig. 23); the whole tube has, therefore, 

 nearly the shape of a retort^ and lies entirely in the lateral 

 part of the brain. The wall of the tube is everywhere formed 

 by a layer of clear, nucleated cells. In the outer limb these 

 cells form a fairly regular columnar epithelium one cell thick, 

 which becomes less regular as the inner limb is approached. 

 The cells bounding the inner limb are arranged irregularly, and 

 they appear to send out processes from their peripheral extremi- 

 ties, which may be supposed to communicate with the pro- 

 cesses of adjacent nerve-cells. The cells of the inner limb also 

 secrete a deep black pigment, which lies in that portion of each 

 cell which is turned towards the lumen of the tube. A clear 

 coagulum sometimes lies in the cavity of this sense-pit. These 

 organs are visible as two black spots at the level of the brain 

 in the dissected animal (fig. 3). 



No trace exists in this genus of the curious finger-like pro- 

 cesses which project from the brain of Sipunculus into the 

 body-cavity. 



Three pairs ofnerves are given off from the brain : (1) 

 dorsally, a small pair supplying the skin of the prseoral lobe — 

 these lie nearest to the middle line (fig. 26) ; (2) ventrally, a 

 nerve on each side, going to the corresponding area of the 

 lophophor, and supplying a branch to each tentacle (fig. 24) ; 

 (3) and posteriorly on each side arises a nerve which passes 

 round the oesophagus, and joins its fellow of the opposite 

 side to form the ventral cord (fig. 24). The lophophoral 

 nerve arises between the point of origin of the nerve of the 

 prseoral lobe and the exit of the circumoesophageal commis- 

 sures. 



The ventral cord itself shows no trace either of a division 

 into two halves, or of a segregation of its nerve-cells into 

 ganglia. It runs along the ventral surface of the body as a 

 perfectly uniform filament, terminating posteriorly without 

 any ganglionic swelling such as that found in Sipunculus. 



VOL. XXXI, PART I. NEW SER. B 



