58 studies on Arthropoda. 1. 



parts are used by the living animals as a stridulating 

 organ. 



Many years ago I discovered casually a highly developed 

 stridulating organ in a species of Ovalipcs (family Portunidse), 

 and afterwards a fine organ in the genus Acanthocarpus , belong- 

 ing to the family Calappidse in the tribe Oxystomata. The aim of 

 the present small treatise is not only to describe these new organs, 

 but to mention the structure in forms of the genus Ocypoda, to 

 consider the curious occurrence of organs both in terrestrial 

 forms, in amphibious Decapoda and in real marine species, 

 two among them inhabiting even moderately deep water, 

 finally from comparison with other classes of animals to point 

 out analogies and the need of further biological observ^ations 

 of living Decapods. 



II. Organs in the genera Ovalipes and Acanthocarpus. 



I. Ovalipes {Platyofiichus) ocellatus Herbst. — On the lower 

 side of each half of the carapace and considerably from the 

 antero-lateral margin is seen a long, narrow, slightly raised area, 

 which is parallel with the direction of that margin and con- 

 sequently arcuate; it begins somewhat from third maxillipeds 

 and reaches to outside the insertion of the cheliped. This area 

 has a large number of rather coarse, transverse ridges completely 

 similar to each other in breadth and extremely regularly arranged, 

 radiating in the direction of the postero-exterior angle of the 

 coxa of the cheliped. The area is broadest near the middle and 

 tapers towards both ends and especially towards the anterior 

 end; consequently the ridges towards this end are gradually 

 shorter, and those near the end only oblong, small tubercles. 

 The part acting as bow is found on the merus of the chelipeds, 

 the upper side of which has proximalh* in front an oblong area 

 with ver>' regular oblique ridges — in a large female to hand 

 ■8 ridges — and these terminate as slight protul)erances on the 



