CRANIAL GANGLIA. 95 



thoracic segments, but they quickly disappear. The flabellar placode finally 

 unites with the base of the sixth leg and then appears to be a part of it. The flabellum 

 lies in the mouth of the channel leading to the gill chamber, and practically all 

 the water going to the gills, either from the front or from the sides, must pass over 

 its anterior surface. This surface is pigmented and very richly supplied with 

 sense organs and nerves, and it undoubtedly serves to test the quality of water 

 going to the gill chamber, (see p. 113.) 



The flabellar placodes are probably represented in scorpion by the lateral 

 coxal sense organs. (Figs. 15-16, s.so.) 



The Cranial Ganglia. — The base of each pedal nerve, near its origin 

 from the brain, enlarges to form an immense spindle-shaped ganglion. Similar 

 ganglia are present in Branchipus, scorpions, and spiders, and they are also present 

 on the pedal nerves of many other arachnids and phyllopods. 



In Limulus they arise, at an early period, from large ectodermic thickenings 

 between the base of the legs and the corresponding neuromere. How the con- 

 nection between the ganglion and neuromere is established was not determined, but 

 it is certain that the ganglion is not an outgrowth of the nerve cord. The body of 

 the ganglion separates from the thickening, but retains its connection with the 

 overlying ectoderm by ganglionated nerve strands. The latter become the 

 gustatory nerves, and the ectodermic remnant of the thickening becomes the 

 mandibles with their gustatory spines. The thick mass of slime buds on the 

 inner face of the mandibles also arises from these thickenings. 



In young Limuli, the ganglia are relatively large masses of cells, separated 

 by clear fibrous stalks from the corresponding neuromere. (Figs. 36-39.) In 

 the adult (Figs. 70 and 218) they are drawn a little closer to the brain, but are 

 never completely merged with it. 



In scorpion embryos, similar ectodermic thickenings appear at the base of the 

 thoracic appendages, furnishing the anlage for the coxal ganglia. (Fig. 74, D.) 

 The thickenings on the third and fourth appendages become greatly enlarged to 

 form the four hypostomeal spurs that lie on either side of the mouth and rostrum. 

 (Figs. 15-16.} The median face of these spurs is highly sensitive (gustatory?) 

 and from them are developed enormous ganglia and thick masses of mucous glands 

 or slime buds. 



In the adult, the median face of the anterior pair of spurs is deeply grooved. 

 The two grooves lie close together and thus form a thick chitenous tube, lined with 

 sensory hairs. In feeding, the scorpion thrusts the spurs into its prey and sucks 

 the blood and other fluids through this tube into the mouth. (Fig. 43, mxl.) 



The independent origin of the flabellar and coxal spur placodes of Limulus 

 and the scorpion suggests their homology with the inner and outer branches of 

 an originally triramous appendage. 



The coxal placodes represent the supra-branchial placodes of vertebrates. 

 Their homology is indicated by their similarity in position, in function, in devel- 

 opment, and, so far as may be determined, in number. In both cases, the pla- 



