400 CONTRIBUTIONS TO MARINE r.IONOMICS. 



will be found that the angle at which the propodite has been arrested 

 by the carpal spine is precisely the angle required for the proper 

 apposition of cheliped to carapace in connection with the respiratory 

 process. The carpal spine acts then as a stay or barrier to excessive 

 flexion of the cheliped. Its function corresponds, therefore, in part to 

 the function of such skeletal processes as the olecranon of the human 

 ulna, which prevents excessive extension of the arm. Examination of a 

 series of Portunids reveals that the variations in the form of the carpal 

 spine in different species and genera are all functionally correlated with 

 the different shapes and proportions of the carapace, and of the segments 

 of the cheliped in the forms examined ; the result in all cases being 

 that the shape of the carpal spine is adapted to ensure the due amount 

 of flexion of the cheliped for the completion of the respiratory channel 

 between cheliped and carapace. 



A similar function seems also to be discharged by the enlarged posterior 

 spine of the antero-lateral margins in Balkyncctes longipes, since the car- 

 popodite presses upwards against it during flexion of the cheliped. An 

 examination of preserved specimens of the Mediterranean lAopa hasiata, 

 and of the American Callinedes sapidus, in which the posterior spine is 

 greatly elongated, seems to me to support this view, though I do not 

 regard the evidence in this case as altogether unequivocal. A complete 

 explanation of the enlargement of this posterior antero-lateral spine 

 should also throw light on the great epibranchial spines of the Oxystome 

 genus Matuta, and of the Lencosiid genera Iphis and Ixa. In the latter 

 cases any relation between the development of the spines and the forma- 

 tion of an inhalant chamber between cheliped and carapace is precluded 

 by the known course of the afferent current in a gutter running between 

 the pterygostomial plate and the exopodite of the third maxilliped. 



The phenomena presented by the respiratory processes of these sand- 

 burrowing crabs throw light, as it seems to me, not only on the problem 

 of the utility of a number of morphologically trivial, but systematically 

 important features of Decapod Crustacea, but also on an altogether 

 different problem, viz., the phylogeny of the r>rachyura Oxystomata. 

 Crabs of the latter group are all characterised by their sand-burrowing 

 habits of life. Similarity of habits often induces homoplastic changes 

 of form in types genetically distinct; but there are certain significant 

 details of structure in the different Oxystome types which appear to me 

 to be only explicable on the view that these crabs are descended from 

 ancestors in which the form of the body closely resembled that of sand- 

 burrowing (Jyclonietopa in being provided with antero-lateral serrated 

 margins, and in which the chelipeds were employed for the production 

 of an extensive inhalant channel, completely roofed over by the pro- 

 jecting teeth of the carapace. For a fuller discussion of this subject I 

 must refer the reader to another paper to be published in the Quarterly 

 Journal of Microscopical Science (1897). 



