Next on the list comes another swimming species, Hombronia depressa, known from 22 species of fishes. Other Hymenosomids prove to be of greater importance than hitherto suspected, making up in numbers what they lack in size. Cancer novae-zelandiae is a benthic crab known from 19 species, and the severe toll taken by fishes suggests a reason for the failure to establish a European congener in New Zealand waters after several years' trial at the station.

Petrolisthes was found in 9 species of fishes, but not frequently and not in large numbers, so that their importance is less than might be inferred from the number of entries. Both species are seclusive, hiding under stones and clinging tightly when disturbed; and their relative infrequence in fish stomachs, in spite of their extreme abundance on shallow bottoms, confirms the inference that these cryptozoic habits, in conjunction with their flattened shape and inconspicuous bluish-grey colours, are of protective value in an environment which proves precarious for less effectively concealed species. A case in point is that of Hemigrapsus crenulatus, a very abundant species in estuaries and on reefs, more venturesome in habits, and although active and armed with massive chelae, not provided with any noteworthy means of concealment; it is eaten in great quantities, notably by red cod from the harbour, and ranks very high as a source of food for fishes. The fact that it is known from only 8 species of fishes emphasises the contrast with Petrolisthes, and shows how a list of entries could be fallaciously interpreted if not qualified by quantitative data. The suggestion that Hemigrapsus crenulatus defends itself by its activity and strength and by its strong chelae is somewhat strengthened by the relative immunity of its congener P. sexdentatus from attack, save by a few of the larger fishes. This crab is known only from four species, and is characterised by its large size, unusually formidable chelae, and bright red colour; possibly the conspicuous pigmentation, in contrast to that of its drab mud-coloured relative, serves as a warning device.

It would appear that swimming habits (Hombronia, Nectocarcinus) imply a heavy toll and a dependance for racial survival on prolific breeding, as in other relatively small organisms of the nekton (sprat, pilchard, whalefeed) and of the plankton (Copepods); that relatively defenceless benthic crabs (Halicarcinus, Hymenicus, Cancer) also require in a lesser degree a high fertility in order that the species may survive; that cryptozoism and concealment colours (Petrolisthes) may have genuine protective value; that bulk and massive weapons (Leptomithrax, Jacquinotia) may give partial or even complete immunity from attack; that these defences may be effectively supplemented by warning colours (Hemigrapsus sexdentatus); and that there is a refuge from many species of predators in estuaries (Helice crassa, Hemiplax hirtipes) and at similarly higher levels on the beaches (Cyclograpsus lavauxi). Without closer discrimination of species it is difficult to determine the functional value of the supposedly protective mask of weeds (Paramithrax minor, P. longipes) or hairs (P. ursus) or sessile animals (Leptomithrax spp.) characteristic of the Majoid crabs.