Tetractinellida appear to flourish best in moderate depths from 50 to 200 fathoms, but they are found to be fairly abundant also in shallower water right up to the coast line, and in deep water up to and beyond the 1000 fathom line. Occasionally they lie free on the bottom, but are far more commonly attached; fixation may be direct or by means of rooting spicules; the occurrence of a stalk is rare. There is great variety in the root tuft, which may be a long loose wisp of grapnel-headed spicules, as in many species of _Tetilla_, or a massive tangle, as in _Cinachyra barbata_; in these cases the sponge is merely anchored, so that it rests at the level of the surface of the ooze; in other cases, _e.g._ _Thenea wyvillei_, the root tuft consists of a number of pillars of spicules which raise the sponge above the level of the ooze, into which they descend and there become continuous with a large dense and confused mass of spicules. The parachute-like base of _Tetilla casula_ invites comparison with the "Crinorhiza" forms of some Monaxonids (p. 216). Two Orders are distinguished thus:— I. CHORISTIDA.—Tetractinellida with quadriradiate spicules, which are never articulated together into a rigid network. II. LITHISTIDA.—Tetractinellida with branching scleres (desmas), which may or may not be modified tetrad spicules, articulated together to form a rigid network. Triaene spicules may or may not be present in addition. ORDER I. CHORISTIDA. _Plakina monolopha_, from the Adriatic and Mediterranean, furnishes a connecting link between the Rhagon stage and other Tetractinellida. The choanosome is simply folded; there is no distinct ectosome; the chambers are eurypylous. The skeleton {213}consists of microcalthrops and their derivatives. The hypophare is well developed. _Plakina_ thus shows a certain amount of resemblance to _Oscarella_ (p. 196), with which it shares the very remarkable possession of flagellated pinacocytes. One of the species of _Tetilla, T. pedifera_, continues the series. The folds of its choanosome are more complicated than in _P. monolopha_, and their outer ends are bridged together by a thin layer of ectosome (cf. species of _Sycon_ among Calcarea); the chambers are still eurypylous. The skeleton reaches a high level: it includes oxeas and triaenes radiately disposed and microscleres (sigmata) scattered throughout the dermal layer. The British _Poecillastra compressa_ from the north of Scotland and Orkney and Shetland is at about the same stage of development, being without cortex and having eurypylous chambers, but it is not so good an example, as the folds of its choanosome are confused. [Illustration: FIG. 105.—Diagrammatic vertical sections of A, Rhagon; B, _Plakina_; C, _Tetilla pedifera_.]