400 CONTRIBUTIONS TO MARINE BIONOMICS. 
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 Bathynectes longipes, since the car- 
popodite presses upwards against it during flexion of the cheliped. An 
examination of preserved specimens of the Mediterranean Lupa hastata, 
and of the American Callinectes sapidus, in which the posterior spine is 
sreatly 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 Jphis and Iza. 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 Brachyura 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 Cyclometopa 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). 
