In _Culeolus Murrayi_ there is a remarkably abundant supply of blood-vessels to the tunic, and these send special branches to a number of small papilliform processes on its outer surface. This system of highly vascular processes probably constitutes, as Professor Herdman suggests, an additional or complementary respiratory apparatus. All these modifications of the branchial system are of particular interest, for we find so many instances of a similar kind among the inhabitants of very deep water. I need only refer here to the modifications of this system in the Isopod _Bathynomus_ already referred to (p. 129), and to the reduction in the number of the gills of many of the deep-sea fishes. Why there should be such modifications is a question upon which the physical and natural history investigations of the conditions of life in the great depths of the ocean at present throw no light. In a previous chapter I have referred to the fact that many of the bathybial animals are characterised by being stalked. Among the simple Ascidia we find many examples of stalked kinds living in deep water, such as _Culeolus_ and _Fungulus_, but also several exceptions, such as _Bathyoncus_, _Styela bythii_, and _Abyssascidia_, that are sessile. It is a noteworthy fact, however, that the genus that has the most deep-sea species—namely, _Culeolus_—is a genus that is provided with a very long stalk. Furthermore, the only known stalked forms of the very large family Ascidiidæ are the abysmal genera _Corynascidia_ and _Hypobythius_. [Illustration: FIG. 20.—_Hypobythius calycodes._ G, nerve ganglion; H, heart; M, the position of the atriopore. The large opening on the upper side is the mouth. (From a drawing by Professor Moseley in Herdman’s ‘Tunicata of the “Challenger” Expedition.’)] The most remarkable character of the genus _Hypobythius_ is the simple condition of its branchial sac, reminding one of the structure of this organ in the shallow-water genus _Clavelina_. ‘There are no folds and there are no internal bars,’ to quote the description given by Professor Herdman; ‘only a single system of vessels can be recognised, branching and anastomosing so as to form a close network, the small rounded meshes of which are the stigmata. The tentacles and dorsal lamina cannot be made out.’ Among the compound Ascidians only four families extend into the abysmal zone, namely, the Botryllidæ, Polyclinidæ, Didemnidæ, and Cœlocormidæ, and of these only one species, _Pharyngodictyon mirabile_, of the family Polyclinidæ, extends into water of greater depth than 1,000 fathoms. In _Pharyngodictyon_ we find the same curious simplification of the branchial sac that we have just referred to in the genera of simple Ascidians, _Culeolus_, _Fungulus_, and _Bathyoncus_. _Cœlocormus Huxleyi_ from a depth of 600 fathoms is a very peculiar form and the type of a separate family, the Cœlocormidæ.