STUDIES ON THE ARACHNID KNTOSTERNITE. 227 



three forms belong to a group distinct from and more 

 specialised tliau that of polyphemus, as is clearly shown 

 by the structure of the genital operculum, etc. Also the 

 pair of posterior ventral apophyses found in polyp hem us 

 are missing in the two young specimens of tridentatus 

 and rotundicauda I have examined. In all species of 

 Limulus the upper side of the entosternite is furnished 

 laterally behind the middle with a short muscle-bearing 

 excrescence, suggesting a suppressed or undeveloped apo- 

 physis.^ 



2. The Entosternite of Scorpions. 



The variations that affect the entosternite of Scorpions 

 are principally correlatetl with the compression, antero- 

 posterior or lateral as the case may be, of the exoskeletal 

 metasternite. In Palamnasus thorelli the ''body" of 

 the entosternite consists of an irregularly transversely oblong 

 plate. From its anterior angles rise the anterior cornua, 

 which give off muscles to the appendages along their outer 

 edge, and present a frayed or ragged appearance when cleared 

 of these tissues (see fig. 20, PI. 14). On its underside the 

 plate dips down on each side of the nerve-cord, and passing 

 and fusing beneath it forms a complete and rigid ventral 

 ring through which the nerves pass backwards into the 

 mesosoma. The lower portion of this ring gives off in front 



' In his paper ou the anatomy of Limulus polyplienius ('Trans. Linn. 

 Soc.,'x.xviii, 1S73), Owen stales (|). 4G9) that the entosternite of this species 

 is furnished with a pair of " sclerous processes " which diverge from " near 

 tlie fore-part of the dorsal surface," and reference is made to H'j^. 5 on pi. 

 xxxviii, which is an acknowledged copy of Van der Hoeven's figure of the 

 entosternite of L. nioluccanus. Yet the original flgures with wjiich Owen's 

 paper is illustrated are all, apparently, taken from examples of the American 

 form (L. polyphemus). Hence it is ditDcult to account for his overlooking 

 the presence of the two pairs of processes in this species. It may also be 

 remarked in passing, though the lapsus is of no great moment, that the 

 statement in the foot-note to p. 462 that " llie species which he [Van der 

 Hoeven] dissected was the rapier-tailed Molucca crab (Limulus rotundi- 

 cauda, Latr.) " is an error. 



