﻿EMBRYOLOGY OP LIMULUS POLYPHEMUS. 219 



In the trilobed character of the body-rings with freedom of movement between each of 

 the several segments we see recalled to life its remote ancestors Neolimulus of the Silurian 

 epoch (PI. XXXI, fig. S), and Bellinurus (PI. XXXI, figs. 1, 3, 4) of the Coal period. 

 The presence of nine segments lends further confirmation to the correctness of our views, 

 already so often expressed in the earlier parts of this Monograph and elsewhere, that we 

 have evidence here not only of the entire thoracic series, but also of a rudimentary 

 abdomen of which the ninth (the telson) represents the hinder somites arrested in their 

 development, as indicated by the like condition of the post-thoracic series in Hemiaspis 

 (see Woodcut, fig. 64, p. 177), and in the tendency to divide up into segments seen in the 

 telson of Stylonurus (see Woodcut, fig. 73, p. 200). 



" The trilobate character of the surface of the body, says Dr. Packard, is very marked, 

 the cardiac or median region being considerably raised above the sides and on the thorax 

 and abdomen ; the sutures do not run continuously across the tergum, but are interrupted 

 at the edge of the cardiac region, thus making the latter more distinct. The sutures 

 between the segments are sinuate, as in the trilobites. The outer edge of each segment is 

 convex, the convexity being greatest on the hinder portion of the edge as in the Trilobites. 

 The branchigerous thoracic feet are still very small, and the median sinus between each 

 is greater in the first than in the second pair. The maxillipeds are much as in the larva, the 

 coxal joint being furnished with a stout spine directed inwards ; but it is not until after 

 several moults that the numerous spines which form the masticating surface appear. 



" The chelae are acute and long, the six spatulate spines on the penultimate joint of the 

 sixth pair in the adult being now represented by simple spines, and the terminal forceps 

 being quite complete. 



" Just behind the last pair of maxillipeds are a pair of oval rather flat tubercles ; their 

 bases are united on the median line of the body, thence diverging outwards. These 

 tubercles do not appear much before this period of the life of the embryo. They have 

 no connection with the last pair of maxillipeds, nor can they be, as Savigny supposed, 

 the rudiments of a seventh pair of legs, otherwise they would have appeared earlier in 

 embryonic life. They are seen in Pigs. 8, 9, and 11 {ch.), PI. XXXIII, closing up the 

 posterior end of the oral groove. 



" In adult life they are movable at their bases and armed on their inner side with 

 recurved spines, like those upon the maxillipeds ; hence they serve the purpose of com- 

 pleting the circumoral armature, and also of retaining the food undergoing trituration 

 preparatory to deglutition. Savigny considered them to form a sort of inferior lip." We 

 are in accord with Professors Huxley, Hall, Owen, and others in considering these as the 

 homologue of the broad heart-shaped metastoma or post-oral plate (Woodcut, fig. 71, 

 p. 180) which closes the buccal aperture behind in the Eurypterida (see Parts I — IV of 

 this Monograph). 



Its great development in Pterygotus seems correlated with the difference in the form 

 and functions of its jaw-feet as compared with those of Limulus. 



