The free-swimming Tunicata included in the group _Ascidiæ salpiformes_, which contains the genus _Pyrosoma_, and the order Thaliacea containing the salps, are in all probability mainly confined to the surface waters. A few specimens of _Pyrosoma_ were captured by the ‘Challenger’ dredges which came up from very deep water, but it is doubtful at what point in the journey to the surface the specimens entered the net. The most remarkable form of free-swimming Tunicate that has come to light is _Octacnemus bythius_, a form that is probably allied to _Salpa_. It was found twice, once in the dredge that came from a depth of 1,070 fathoms, and once from 2,160 fathoms. The tunic of the animal is gelatinous and hyaline, but the most important feature it possesses is an imperforate membrane separating the branchial sac from the peribranchial cavity. Octacnemus, in other words, possesses no true stigmata, these structures being represented only by little pits in the walls of the branchial sac. This curious and extremely interesting modification of the respiratory organs points very strongly to the conclusion that Octacnemus is truly a deep-sea animal. CHAPTER VIII THE FISH OF THE DEEP SEA Of all the groups of animals that constitute the deep-sea fauna, the fish show the greatest number of peculiarly abysmal characters. Being much more highly differentiated than the invertebrates, they possess more organs liable to undergo modifications of colour, size, and structure, and consequently we are able to point to a great many more features characteristic of deep-sea fish than we can do in any other group of animals. The first point that calls for remark in the consideration of the fish fauna of the deep sea is the almost entire absence of ancient and primitive types. The Elasmobranchii, including the Sharks, Rays, and Chimæra, constituting the order that from anatomical embryological grounds is always regarded by naturalists as the most primitive order of this class, is represented in very deep water by only one species. _Raia hyperborea_ and _Chimæra monstrosa_, it is true, just enter into the abysmal zone, but _Chimæra affinis_ is the only Elasmobranch that extends to depths of over 1,000 fathoms. The Ganoidei too, the order that in palæozoic and mesozoic times was so rich in genera and species, is entirely absent from the abysmal zone, not a single representative having been found at any time by any of the deep-sea expeditions. The Dipnoi, that remarkable order including the three fresh-water genera, _Ceratodus_ from Australia, _Lepidosiren_ from Brazil, and _Protopterus_ from West Africa, has no representative and no ally in the deep waters of the ocean.