ON THE ORIGIN OP VERTEBRATES FROM ARACHNIDS. 327 



Vertebrates six sets of sense-organs^ or exactl}' the same num- 

 ber as in Scorpio and Limulus, In Vertebrates the ganglia, 

 presumably derived from segmental sense-organs, are the ciliary, 

 Gasserian^ facial, auditory, and glossopharyngeal, and the first 

 free vagus ; or omitting the latter, and counting with some 

 authors the facial ganglion as double, we would still arrive at 

 the same conclusion. (3) In both Scorpio and Vertebrates these 

 sense-organs give rise to '' lateral ganglia/^ (4) In both cases 

 neural ganglia are developed in the head, which are serially 

 homologous with spinal ganglia. (5) In both cases the neural 

 and the spinal ganglia develop from a special modification of 

 the edge of the nerve- cords, the " neural crest. '^ In Scorpio the 

 crest consists of a row of large dark sense-organs extending the 

 whole length of the nerve-cord (Pis. XXIII and XXIV, figs. 

 1 — 3, nc. = sp. y. ; and text, Fig. 2, a and d, nc, and 5/j. g. ; also 

 text, Fig. l\,S2).g.). (6) The manner in which the coxal nerve 

 unites with a coxal sense-organ and receives ganglion-cells 

 from it, and the way it becomes connected by small branches 



Fig. 3. — Semidiawrammatic section tiirough the base of a leg aud a 

 thoracic neuromere of an embryo Scorpion. — a, h. n. Anterior hgenial 

 nerve, ex. s. o. Coxal sense-organ, ff. s. s. Ganglion-cells arising from 

 segmental sense-organ, n. g. Large neural ganglion, serially homologous 

 with spinal ganglia of abdomen, p. h. n. Posterior hfemal nerve. /;. ti. 

 Pedal nerve, s. b. Sensory buds (comp. fig. 4, PI. XXIV, s. o".), s. s, o. 

 Segmental sense-organs. 



