THE DICEPHALON AND MICROCEPHALON. 



arches persist as the circumoral, visceral arches of vertebrates, that is, as the 

 pre-maxillary, maxillary, mandibular, and hyoid arches, and possibly the first 

 gill arch. (Figs. 32-34, 160-172.) 



Taste Buds, Slime Buds, ami Cranial Ganglia. In the typical appendicular 

 arches of arachnids, there is a lobe on the median or neural side that forms the 

 mandibular or coxal spurs, and in which are located important groups of sense 

 organs, i.e., gustatory buds and slime buds. They are the forerunners of the 



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o -y 



if 



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FIG. ii. FIG. 12. 



FIG. ii. Diagrams of marine arachnids, to illustrate the relations of their organs to those in the ostracoderms. 

 FIG. 12. C, hypothetical form, intermediate between a merostome and an ostracoderm (Cephalaspis) ; D, is 

 an accurate restoration of a small cephalaspid (sp. nov. ?) from Scaumenac Bay, P. Q., except the external gill, 

 r.v. g. which are hypothetical. 



" epibranchial organs," "lateral line organs," and "gustatory organs" of verte- 

 brates. At an early embryonic period, in the wide zone between the nerve cord 

 and the coxal and gustatory spurs, and in close connection with the latter, immense 

 oblong ganglia (pedal ganglia) are developed from thickenings of the overlying 

 ectoderm. (Figs. 36-39, 134-137.) These ganglia arise independently of the 

 medullary plates. Later, they unite the proximal end of the pedal nerve with the 

 corresponding neuromere. They are the forerunners of the cranial ganglia of 

 vertebrates. 



Segmented Sense Organs. In the scorpion, each appendicular arch, except 

 the first, has, on its lateral margin, close to the base of the coxa, two sensory cups, 

 in form and in minute structure very similar to the conspicuous pits on the outer 

 surface of the neuromeres. (Figs. 15-16, 74, s.so.) All these pits quickly lose 

 their sensory character and later apparently disappear or are converted into gan- 

 glion cells. 



