No. I.] SENSE-ORGANS OF LUMBRICUS AG RICO LA. 211 



approaches the mid-dorsal line, it leaves the space between 

 the two muscle-layers, runs among the circular muscle-fibres, 

 and finally reaches the base of the epidermis near the mid- 

 dorsal line. The dorsal ramus of one side never crosses the 

 mid-dorsal line to the other side of the metamere. The ventral 

 ramus of each nerve-root turns between the two muscle-layers 

 and passes to a point near the mid-ventral line, but never 

 crosses this line (text, Fig. 2). There are thus formed in each 

 segment three nerve-rings, which are incomplete in the mid- 

 dorsal and mid-ventral line and which lie for the greater part 

 of their course between the circular and longitudinal muscle- 

 layers. Each of these nerve-rings is formed of four parts, the 

 two dorsal and the two ventral rami of a pair of nerves. Back 

 of the anterior metameres from which the diagram for text. 

 Fig. 2, was made, an "accessory" tract of longitudinal mus- 

 cles appears between the ventral tract and the inner intersetal 

 tract. Each of the three nerve-roots then passes directly 

 to the circular muscle-fibres through the space between this 

 accessory and the ventral tract, and does not divide into 

 its dorsal and ventral rami until the circular muscles are 

 reached. In this case no ventral branch is given off from 

 the dorsal ramus, and each nerve-ring appears more complete 

 in that each half can be traced directly from the mid-dorsal 

 around to the mid-ventral line, and is not interrupted in the 

 region of the inner intersetal tract, as shown in the diagram. 

 From each of these nerve-rings, the epidermal nerves before 

 noted pass to the epidermis, and as they are given off the 

 dorsal and the ventral rami of the nerve-rings become smaller. 

 The epidermal nerves leave the nerve-rings at various angles 

 but are generally inclined cephalad ; they rarely approach the 

 epidermis directly at the base of a sense-organ, but they are 

 most numerous in those regions in which the sense-organs are 

 most numerous. The epidermal nerves from the anterior nerve- 

 ring supply the epidermis of the anterior part, probably almost 

 half, of a metamere ; those from the middle nerve-ring supply 

 the epidermis in the zone which includes the setae, and those 

 from the posterior ring supply the posterior part of the meta- 

 mere. 



