102 PROCEEDINGS OP THE AiMERICAN ACADEMY 



which is rudely spherical in shape. A bundle of muscle fibres (PI. II. 

 fiof. 11, b. 771U.) passes from its posterior face upward and backward to 

 the body wall. This bundle, together with a few fibres passing from 

 its anterior face upward and forward, serves to support the ganglion 

 in place, and perhaps to move it slightly. The ganglion is composed 

 of a central fihroiis part, surrounded on all sides except the lower by a 

 hiyer of nerve-cells from one to four cells deep. On the lower surface 

 the fibrous part of the ganglion is exposed. 



Biilow ('83, p. 75) has described six pairs of nerves arising from 

 the corresponding ganglion and commissure in Lnmhriculus variegatus. 

 In Aulophorus I have found only four pairs, three from the ganglion, 

 and one from the commissure. The first pair (PI. 11. fig. 10 ; PI. III. 

 fig. 26, li^.) corresponds to the sense nerve of Biilow. Each arises 

 from about the middle of the anterior face of its half of the ganglion, 

 and passes to the extreme anterior end of the segment. It is there 

 distributed to the body walls. In Lumbriculus, Biilow ('83, p. 75) 

 describes a group of cells in the anterior end of the first segment. 

 These cells constitute a so-called se;ise organ, and to them this nerve 

 is distributed, and is therefore called by him a sense nerve. Cross 

 sections of Aulo[>horu3 show apparently similar large cells in the same 

 position, but longitudinal sections (compare PI. II. fig. 11) show these 

 to be nothing more than sections of the swollen parts of the muscular 

 cells whose processes pass to the walls of the pharynx. 



The second and third nerves from the supra-oesophageal ganglion 

 (PI. III. fig. 25, TO^, n*) arise together, lower and farther from the 

 median plane than the first nerve. A short distance from their origin 

 they separate, one passing to the anterior walls of the segment, and 

 the other to the upper lip and adjacent parts. 



The fourth nerve (PI. III. fig. 25, ii^) arises from the commissure 

 and passes to the lower lip. No nerve has been found passing from 

 the supra-oesophageal ganglion to the lateral line, nor has more than 

 one been found to arise from the commissure. 



The oesophageal commissures pass obliquely backward from the 

 brain to the ventral cord in the anterior part of the first bristle-bearing 

 segment. They are composed principally of fibrous nerve substance, 

 but there are a few ganglionic cells on their surfaces, except for a 

 short distance in the middle part of their course, where the nerve cells 

 are entirely wanting. 



The ventral cord passes backward from the point of union of the 

 two commissures to the region of the pavilion, where it gradually 

 merges into the terminal germ-zone (PI. III. fig. 31). In cross sec- 



