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 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.
Fig. 105.—Diagrammatic vertical sections of A, Rhagon; B, Plakina; C, Tetilla pedifera.
From T. pedifera we pass to the other species of Tetilla and all the higher genera of Choristida; these possess a cortex not of homologous origin in the various cases, but probably to be classified under one of two heads, typified by Stelletta and Craniella respectively (Fig. 106).
Fig. 106.—A, Craniella type; B, Stellettid type. ch, Chone; co, collenchyma; d.o, dermal ostia; fb, fibrous tissue; i.c, intercortical cavity; sd, subdermal cavity; sp, sphincter. (After Sollas.)