ANATOMY OF ARENICOLA ASSIMILIS. 749 



surrounded by clusteis of nerve-cells and neurogliul tissue. 

 Bundles of nerve-fibrils may be traced from tlie bases of the 

 prostomial epithelial cells into the neuropile (see fig. 23). 

 Larger unipolar ganglion-cells are found immediately out- 

 side the neuropile, and particularly on the side nearest the 

 middle line. The oesophageal connectives arise from the 

 anterior lobes at the point where the neuropile reaches its 

 greatest development (fig. 22). The eyes are found on the 

 dorsal side of this part of the brain. There are four or five 

 on each of the anterior lobes. A little further back large 

 pyriform ganglion-cells become more numerous, and 

 especially on the inner side of the two anterior lobes just 

 before they unite in the middle line and for some distance 

 after their union (see fig. 23). Passing backwards along 

 this region, it is seen that the ganglion-cells become more 

 restricted to the dorsal and lateral faces of the brain, the 

 middle and ventral parts being composed largely of neuro- 

 pile, in which also neuroglial cells and fibrilke may be 

 recognised. The ganglion-cells of this region are more 

 intimately associated with the median part of the prostomium. 



The posterior brain-lobes are small and tapering, and 

 gradually merge into two nerve-tracts which lie cm the inner 

 side of the nuchal organ just below the sensory epithelium. 

 The brain of the specimen 120 mm. long is 0'G5 mm. in 

 length, 1 mm. broad across the anterior lobes, and about 0"4 

 mm. deep in this region. It is most nearly like the brain of 

 A. marina, but is wider anteriorly. (The brain of a speci- 

 men of A. marina 120 mm. long is about 0"7 mm. wide.) 



OEsophageal Connectives. — The oesophageal connec- 

 tives arise from the lateral region of the broadest part of the 

 anterior cerebral lobes, i. e. just in front of their point of 

 union. Each is a stout fibrous cord with numerous cells ou 

 its outer face, which, in the first part of its course, lies about 

 a millimetre below the epidermis, and is slung up in a 

 muscle sheet, which is attached to the subepidermal muscu- 

 lature by numerous muscle-strands (fig. 22). It is only in 

 the ventro-lateral region that the connectives approach the 



VOL. 4G, I'AKT 4. NEW SKIUES. CCC 



