Here then we are presented with a veritable puzzle for which we can at present frame no manner of answer. Pollicipes on the one hand—like Lingula among the brachiopods—has been able to maintain itself almost unchanged amid the tremendous struggle for life of the shallow water of the tropics ever since the Lower Oolite epoch; while Scalpellum, on the other hand, has either become profoundly modified, or been driven into the abysmal depths of the ocean. The group of the Thoracostraca, or stalk-eyed Crustacea, including lobsters, crabs, hermit crabs, prawns, and shrimps, is well represented in the deep sea. Most of them are characterised by being quite blind (in many cases even the eye-stalks are obliterated), by being protected with a dense covering of spines, by the thinness of their shells, and by their bright red or carmine colour. The order Stomatopoda is almost entirely confined to the shallow waters of the tropical or temperate shores. Not a single species is known to inhabit the deep sea, and only a very few specimens have been captured in more than a few fathoms of water. The Schizopoda, however, present us with many curious abysmal forms. Most of the genera of this order belong to the pelagic plankton, and many of them are known to possess the power of emitting a very strong phosphorescent light. Several genera, however, such as _Gnathophausia_, _Chlaraspis_, _Eucopia_, _Bentheuphausia_, &c., never seem to leave the great depths of the ocean, and nearly all of these genera are distinguished by being quite blind or possessing very much reduced or rudimentary eyes. [Illustration: FIG. 16.—_Euphausia latifrons_, from the surface of the sea. (After Sars.)] [Illustration: FIG. 17.—_Bentheuphausia amblyops_, from 1,000 fathoms. (After Sars.)] If we compare, for example, the pelagic _Euphausia latifrons_ (fig. 16) with the nearly allied but abysmal _Bentheuphausia amblyops_ (fig. 17), the difference in this respect between a Schizopod living in the sunlight and one living in the darkness of the deep-sea is very apparent. The pelagic Schizopoda are usually quite pale and transparent; the deep-sea forms on the other hand are frequently if not invariably of a bright red colour, as is the case with many other deep-sea Crustacea to which reference will be made later on. Passing on to the group of the Decapoda, we find that the most interesting of all the abysmal cray-fish is the family of the Eryonidæ; indeed, in some respects the discovery of these curious forms may be reckoned among the most valuable results of the ‘Challenger’ Expedition. They are characterised by the dorsal depression of the anterior part of the cephalothorax, the absence of a rostrum, and the absence or very rudimentary condition of the eyes (fig. 18). Their nearest relations seem to be certain genera of Crustacea that are found in jurassic strata, in the lias, and more particularly in the lithographic slates of Solenhofen.