ON THE 0RIC4IN OF VERTEBRATES PROM ARACHNIDS. 325 



the pedal and haemal nerves of the chelae, but also with the skin 

 (Fig. 1, g. 7\). I believe this ganglion is formed by the fusion 

 of the segmental and coxal ganglia. 



All the pedal nerves of the thorax are built on the above 



Fig. 2. — A. Section through the anterior portion of a neuromere of Scorpio 

 in Stage e, fig. 3. B. Section, at same stage, through the posterior 

 portion of a neuromere. C. Section of nerve-cord in anterior portion 

 of abdomen of a Scorpion embryo, with pigment just appearing in 

 the body-wall. D. Section through the third abdominal neuromere of 

 embryo about ready to hatch. E. Section through first abdominal neuro- 

 mere of adult. F. Section through the occipital ring of the endocranium 

 or sternum of Scorpio. G. A segmental sense-organ of Stages f, g. 

 l/L c. Botryoidal cord. c. cr. Cartilaginous endocranium. c. cent. = g. m. c. 

 Imperfect canalis centralis formed by the ganglionic portion of the 

 median furrow, g. m. c. Ganglionic portion of median furrow, g. s. o. 

 One of the large marginal sense-organs that give rise to the spinal ganglia. 

 k. n. Haemal nerve, i. m. c. Interganglionic portion of median furrow, 

 or " anlage " of spinal artery, i. nl. Inner neurilemma, k. st. Wedge- 

 shaped cord, a remnant of the median furrow, out of which a branch to 

 the spinal artery is formed. «. cr. Neural crest. md. Medulla. 

 n. n. Neural nerve. sp. a. Spinal artery. sp. g. Spinal ganglion. 

 sp. n. Spinal nerve, sx. o. Sexual organs. 



