NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS II5 



lateral lobes of the opisthosoma. From a comparative study of the 

 position of the cardio-aortic valve in the Chelicerata, Petrunkewitch 

 (1922) found that the valve is always between segments VIII and 

 IX, and he therefore claimed that segment VIII is included in the 

 prosoma of Limiilus. His contention is but little affected by the 

 modified view here shown to be in better accord with the facts. The 

 operculum is anatomically more closely connected with the prosoma, 

 from which it derives its innervation, than with the opisthososa, and 

 the partition of the tergum of its segment between the prosoma and 

 the opisthosoma, as above noted, is but a necessary adaptation to give 

 intersegmental action to primarily intrasegmental muscles. 



The Eurypterida and Arachnida differ from the Xiphosurida in that 

 the prosoma includes only six somites, and in this respect they are 

 nearer to the Trilobita, which have only four prosomatic somites. 

 The eurypterids and arachnids, however, lack the lateral expansions 

 of the prosomatic carapace characteristic of the trilobites and xipho- 

 surids, and, judging from the more anterior position of the lateral eyes 

 (fig. 49 E, E), it seems probable that the acronal element of the 

 prosoma is less extensive on the marginal areas of the latter, though 

 medially it must include the region of the dorsal eyes (dO). 



In a typical arachnid embryo (fig. 49 A) the somites are regular 

 transverse sections of the trunk behind the large prostomial acron 

 (Acr), which is produced laterally into a pair of cephalic lobes. 

 Ordinarily there are no appendages on the acron, but Jaworowski 

 (1891) has described a pair of apparent antennal rudiments in a 

 species of Trochosa (C, b), and Pokrowsky (1899) found two pairs 

 of transient precheliceral lobes in an embryo of Pholcus opilionides 

 (B, a, h), the second of which, he says, correspond in position with 

 the "antennal" rudiments described by Jaworowski. The nature of 

 these embryonic lobes may be doubtful, but since the trilobites have 

 well-developed antennae, there is no reason why embryonic vestiges 

 of acronal appendages might not recur in some chelicerate forms. In 

 adult Solpugida there is a pair of small appendages (fig. 49 F, Antf) 

 arising at the sides of the epistomal lobe, which are movable by 

 muscles (G, mcl), and are, therefore, suggestive of being antennal 

 remnants. 



The cheliceral somite of the arachnid embryo (fig. 49 A, /) lies 

 transversely immediately behind the acronal lobes ; but in the adult 

 this somite curves forward around the sides of the labrum from 

 behind the mouth as in Liinnlus, so that the chelicerae come to have 

 a preoral position above the labrum (fig. 48 A, Chi), though usually 

 they are separated by a median epistomal bar extending downward 



