196 BRITISH FOSSIL CRUSTACEA. 



development of Limulus polypthemus, have led them to adopt the same conclusions (see 

 'Development of Limulus polgphemus,' infra). — H. W.]. 



" Seventh Epimeral Nerve (n, 11, PL XXXVI ). — This arises dorsad of the origin of 

 the maxillipedal nerve, passes backward and outward to its proper apodeme (vi), where it 

 divides, and, running onward, ramifies to supply the tissues in the hinder produced 

 angles of the cephaletron. 



"Eighth Epimeral Nerve (n, 12, PI. XXXVI). — This is one of the same system of 

 dorsal nerves, succeeding the seventh ; it passes backward and slightly outward along 

 the dorsal margin of the seventh apodeme (vn), and dips into the articular depressions 

 between the cephaletron and thoracetron. 



" Opercular Nerve. — The hindmost pair of cephaletral ventral nerves [n, PI. XXXIV, 

 and n, viii, PI. XXXV, fig. 1) is given off at or just before 1 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 

 coalescent pair of limb- appendages of the segment anchylosed to the back of the 

 cephaletron. 



"The corresponding dorsal pair of nerves (n 13, PI. XXXVI) 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. XXXIV and XXXV), extends backward about an inch 

 before its first ganglion (§) is formed ; this is followed by three others (e, I, tj), 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 shortening as the 

 chord recedes. 



" Each of the four anterior ganglions gives off" two pair of principal nerves, one dorsal 

 and anterior (n, 14 — 17, PI. XXXVI) the other ventral or posterior (n, ix — xn, Pis. 

 XXXIV and XXXV), 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 backwards before distributing themselves among the tissues of 

 their corresponding abdominal segments. The dorsal nerves (n, 17, PI. XXXVI) 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. 



1 "This lends countenance to the idea that the 'opercular limb,' viii, is the last of the cephaletral 

 series." [See remarks on this plate in Pterygotus, ante, Part I, p. 40, et seq. — H. W.] 



