18 



or coalesced pair of limb-appendages of the segment ancbylosed to the back of the 

 cephaletron. 



The corresponding dorsal pair of nerves {n 13, PL V.) also course along the sides 

 of the ganglionic chord before diverging to ramify in the middle third of the interspace 

 between the cephaletron and thoracetron. 



The ganglionic chord (y, Pis. II a., IV.) extends backward about an inch before its 

 first ganglion (S) is formed; this is followed by three others (e, t, >») and a terminal 

 swelling (9) situated about an inch and a half from the joint of the tail-spine. The 

 interganglionic tracts average in length about 2^ lines, slightly shortening as the chord 

 recedes. 



Each of the fom' anterior ganglions gives off two pairs of principal nerves, one dorsal 

 and anterior {n 14-17, PL V.), the other ventral or posterior {n ix-xii. Pis. II a. & IV.), 

 these terms not being absolute, but meaning the approximate relative position of the 

 places of union of the nerves with the ganglion. The dorsal nerve is a serial repetition 

 of the antecedent epimeral ones. It passes outward along the contiguous apodemal in- 

 terspace, on emerging from which it divides ; the posterior branch quickly subdivides ; 

 the anterior branch continu.es further before subdividing ; all these filaments incline ob- 

 liquely backward before distributing themselves among the tissues of their corresponding 

 abdominal segments. The dorsal nerves {n 17, PL V.) from the fourth ganglion run 

 backward a short way before inclining outward. A filament of each of the thoracetral 

 epimeral nerves can be traced to the movable side-spine of its segment. 



The ventral nerve goes obUquely outward and backward to the space or joint between 

 its own segment and the one in advance, penetrates the branchial leaf-limb at the part 

 or moiety of its own side, distributes many filaments to the basal joint, and is continued 

 on through the second and third joints, before being finally resolved in the fourth and 

 terminal joint (PL II a. n ix-xii). More minute filaments are sent ofi^, usually between 

 the origins of the two chief nerves, from the four ganglions. 



The terminal ganglion (6, Pis. II a. & IV.) represents the coalescence of the nerve- 

 centres of at least three segments. Its anterior dorsal pair of nerves (« 18, PL V.) 

 traverse the interval between the fifth (xii) and sixth (xiii) thoracetral apodemes, in a 

 course more obliquely backward than the antecedent pairs. The corresponding anterior 

 ventral nerves {u xiii. Pis. II a. & IV.) supply the sixth pair of leaf-limbs or fifth 

 branchial pair (xiii). Below the origin of this pair, nervous filaments (r, PL II a.) pass 

 off to the lower fourth of the intestine. 



The second ventral pair {n xiv, ib.) is chiefly distributed to the fibres of the flexor 

 muscles of the tail-spine, arising from and occupying the soft, rather tumid tract, which 

 resembles a leaf-foot soldered down to form the covering of the hindmost part of the 

 ventral surface of the thoracetron. 



The third pau' of principal nerves from the terminal ganglion represents a bifid con- 

 tinuation of the neural axis (PL IV., I). After a course of about three lines, each 

 sends off a nerve {u xv, PL IV.) belonging to the ventral series, which supplies the 

 hindmost or postanal region of the abdomen affording the articular surface for the taU- 

 spine. 



