476. PROF. OWEN ON THE ANATOMY 
margin of the seventh apodeme (vir) and dips into the articular depression between 
the cephaletron and thoracetron. 
. Opercular Nerve.—The hindmost pair of cephaletral ventral nerves (n, Pl. XXXVII. 
& n vi, Pl. XXXVIII.) is given off at or just before * the continuation of the neural 
ring into the *ganglionic chord. The nerves of this pair run along the sides of the 
latter for about 8 lines, then slightly diverge, curve outward, and send off one or two 
filaments laterally, before descending to penetrate the base of the anterior or opercular 
leaf-foot, or coalesced pair of limb-appendages of the segment anchylosed to the back 
of the cephaletron. 
The corresponding dorsal pair of nerves (7 13, Pl. XXXIX.) also course along the 
sides of the ganglionie chord before diverging to ramify in the middle third of the inter- 
space between the cephaletron and thoracetron. 
The ganglionic chord (y, Pls. XXXVII., XXXVIIL) extends backward about an 
inch before its first ganglion (8) is formed; this is followed by three others (e, č, n) 
and a terminal swelling (0) situated about an inch and a half from the joint of the 
tail-spine. The interganglionic tracts average in length about 2; lines, slightly short- 
ening as the chord recedes. 
Each ofthe four anterior ganglions gives off two pairs of principal nerves, one dorsal and 
anterior (n 14-17, Pl. XX XIX.), the other ventral or posterior (n rx—x1r, Pls. XXXVII. 
& XXXVIIL.), 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 interspace, on emerging from which it divides; the posterior branch quickly 
subdivides; the anterior branch continues further before subdividing ; all these filaments 
incline obliquely backward before distributing themselves among the tissues of their 
corresponding abdominal segments. The dorsal nerves (» 17, Pl. XXXIX.) 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 obliquely 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. XXXVII. v 1x-x11). More minute filaments are sent off, usually 
between the origins of the two chief nerves, from the four ganglions. 
The terminal ganglion (0, Pls. XXXVII. & XXXVIII.) represents the coalescence of 
the nerve-centres of at least three segments. Its anterior dorsal pair of nerves (x 18, 
Pl. XXXIX.) traverse the interval between the fifth (x1r) and sixth (xrrr) thoracetral 
apodemes, in a course more obliquely backward than the antecedent pairs. The cor- 
responding anterior ventral nerves (n xir, Pls. XXXVII. € XXXVIII.) supply the 
sixth pair of leaf-limbs or fifth branchial pair (xrm). Below the origin of this pair, 
nervous filaments (r, Pl. XXXVII.) pass off to the lower fourth of the intestine. 
* This lends countenance to the idea that the * opercular limb,’ vrrr, is the last of the cephaletral series. 
