292 Transactions. 



Art. XX. ■ — The New Zealand Sand-hoppers belonging to the Genus 



Talorchestia. 



By Charles Chilton, M.A., D.Sc, LL.D., F.L S., C.M.Z.S., Hector 

 Medallist, Professor of Biology, Canterbury College. 



[Read before the Philosophical Institute of Canterbury, 16th October, 1916 ; received 

 by Editors, 30th December, 1916 ; issued separately, 24th August, 1917.] 



The sand-hoppers or shore-hoppers that are so abundant on the shores 

 of New Zealand are Amphipoda belonging to the family which Stebbing 

 has named the Talitridae, though it was long formerly known, and is per- 

 haps better known even now, as the Orchestidae. They present many 

 interesting points in structure, and are specially worthy of study because 

 they are undoubtedly forms that have arisen from marine ancestors and 

 have gradually become adapted to a life on the seashore. Some members 

 of the family are still almost entirely marine, spending the greater part 

 of their life in the sea-water ; others live at or above high-water mark 

 almost beyond the reach of ordinary tides. Between these two extremes 

 we have numerous transitional stages in structure and in habits, so that the 

 family offers a favourable opportunity for studying the effects of altered 

 conditions on the structure of the animals. One member of the family, 

 Parorchestia sylvicola (Dana), has become quite terrestrial in habits, and is 

 found all over New Zealand far from the coast. 



Similarly the terrestrial Isopoda belonging to the Oniscidae and allied 

 families, commonly known as slaters, woodlice, &c, are descended from 

 marine ancestors and present a series of transitional forms strikingly parallel 

 to the Talitridae, though with the isopods the terrestrial forms are much 

 more numerous and more perfectly adapted to terrestrial conditions. In 

 their antennae and mouth-parts these isopods show many resemblances 

 to the shore-hoppers, and a detailed comparison of the two groups, as an 

 example of convergent evolution, would be instructive and fruitful in results. 

 It is possible also, as I pointed out in 1884 (p. 156),* that forms now 

 included in the same species or genera may have arisen independently in 

 different places from the same widely distributed marine ancestor. 



The New Zealand sand-hoppers are grouped under several genera, such 

 as Talorchestia, Orchestia, &c. Their classification is difficult owing to the 

 close general resemblance of all of them and the consequent use of minute 

 and apparently trivial characters in distinguishing the genera and species, 

 and particularly owing to the fact that nearly all show marked sexual di- 

 morphism, the females of different species being very much alike while the 

 fully grown males may be quite different. Again, the sexual characters of 

 the male appear in most cases to be fully developed only at a late stage, the 

 immature males being more like the females, and in the absence of a mature 

 male it may be very difficult to say to what species any particular specimen 

 belongs. 



The Orchestidae of New Zealand have already received a good deal 

 of attention. The species were originally described by Milne-Edwards, 



* The references are made by the year of publication to the bibliographic list at 

 the end of this article. 



