The Brachiopoda need not detain us long. Some species are capable of existing at a great variety of depths without any observable modification of shape or characters. Thus _Terebratulina caput serpentis_ has the extraordinary bathymetrical distribution of 0–1,180 fathoms, and _Terebratula vitrea_ 5–1,456 fathoms. _Atretia_ is the only genus peculiar to deep water. It is a noteworthy fact in connection with this order that the two genera, _Lingula_ and _Glottidia_, which compose the sub-order Ecardines, are both confined to shallow water. Now the Ecardines are anatomically, at any rate, the most primitive of the Brachiopoda, and Lingula has the most ancient geological history of any living genus of the animal kingdom, shells almost identical with those of the living species being found abundantly in the Cambrian strata. Why it is that Lingula has been able to maintain itself almost unchanged through all the countless generations that have elapsed since Cambrian times, and can now flourish amid the desperate struggle for existence in the shallow waters of the tropics, while its companions, the corals, mollusks, arthropods, &c., have changed or passed away, is one of those problems in natural history that seem to us impossible of solution. The time may come when we shall be able to appreciate better than we do now the complicated relations between animals and their environment, and then perhaps the peculiar fitness of Lingula will be made manifest; but at present we can but mention the fact as a fact, and leave the solution of the problem to the future. The order Gephyrea is probably another very ancient group of animals, although in the absence of any hard calcareous, siliceous, or horny skeleton the geological record can give us no confirmation of their antiquity. As with the Brachiopods so with the Gephyrea, some of the species have a very wide bathymetrical distribution. _Sipunculus nudus_, for example, the commonest and best known of all the Gephyrea, extends from quite shallow water to a depth of over 1,500 fathoms. As Selenka has pointed out, it is those Gephyreans that live in holes in stones, or in shells such as _Phascolion_ and _Phascolosoma_, that are more frequently found at the greater depths; but, apart from this, there are no characters that exclusively belong to the abysmal Gephyrea or are more frequently found in them than in the shallow-water forms. Nor are there any genera, at present brought to light, that are confined to those regions of the sea.