2^6 KINGSLEY. [Vol. VIII. 



the mctastoma (which numerous authors have regarded as an 

 appendage homodynamous with the others) the result will be 

 to bring the third thoracic foot of the Ilexapod into homology 

 with the first rather than the second maxilliped of the Crus- 

 tacea, and there is no little evidence to show that here, if 

 anywhere, the line between head and thorax in the Crustacea 

 is to be drawn. We are, however, more concerned at present 

 with the serial comparison between Limulus and the Arachnids, 

 and the studies of the nervous system warrant the comparisons 

 made above. 



Although we are fully justified in the recognition of somites 

 in both Limulus and Arachnids in front of the segment of the 

 first appendage, I have, from convenience, followed the old 

 nomenclature in the following discussion, and have numbered 

 the somites according to the appendages, somite I being that 

 of the first appendage. From the foregoing it will be seen 

 that point 7, the transfer of appendage I from a postoral to a 

 prestomial position has but little value in deciding the affini- 

 ties of Limulus, while, if our comparisons be correct, point 

 10, the existence of a regional division of the body behind 

 appendage VI occurs in the Hexapods and possibly in the 

 Crustacea as well. Point 35 — the absence of any differen- 

 tiated head — is closely allied to this last. In the Arthropods 

 the terms "head" and "thorax" must be used with physio- 

 logical rather than morphological values. The cephalothorax 

 of Limulus, as well as that of the Arachnids, is co-extensive, so 

 far as our present knowledge goes, with both head and thorax 



upon which he relies to support his statement that " the first antennas are 

 evidently preoral from the very earliest period at which the mouth is visible," 

 represents not the first antennae, but the optic lobes alone. Herrick says ('92, p. 

 442) that he cannot agree with me in saying that Reichenbach " has all the 

 appendages at first distinctly postoral." Reichenbach figures (Fig. "ja) the condi- 

 tion to which I referred. A leader (//;.) goes to the " labrum," a thickening of 

 cells some distance in front of his antennul^e [ES^). His sections show that there 

 is behind this but (if I read his description aright) still in front of the atenn- 

 nulre, a mass of cells, his " Vorderdarmkeim." If this be so I am certainly justi- 

 fied in my reference to Reichenbach as showing the mouth in front of the first 

 pair of appendages. There is as yet no functional mouth found, but the collection 

 of cells indicated by Reichenbach marks the point of the later stomodaeal invagi- 

 nation. 



