As in _Halichondria_, and in fact all sponges, the body-wall is formed of two layers; the gastral layer, as we have said, forming a continuous lining to the Ascon tube and its branches. The {186}dermal layer includes a complete outer covering of pinacocytes, which is reflected over the oscular rim to meet the gastral layer at the distal end of the tube; a deeper gelatinous stratum in which lie scleroblasts and their secreted products—calcareous spicules; and finally porocytes.[216] These last are cells which traverse the whole thickness of the thin body-wall, and are perforated by a duct or pore. The porocytes are contractile, and so the pores may be opened or closed; they are a type of cell which is known only in Calcarea. It will be noticed that the fusiform or stellate "connective tissue cells" are absent. The layer of pinacocytes as a whole is highly contractile, and is capable of diminishing the size of the sponge to such an extent as quite to obliterate temporarily the gastral cavity.[217] The choanocytes show certain constant differences in structure in the families Clathrinidae and Leucosoleniidae respectively. In the former, the nucleus of the choanocyte is basal; in the latter, it is apical, and the flagellum can be traced down to it (Fig. 77). [Illustration: FIG. 77.—The two types of Asconid collar cells. A, of _Clathrina_, nucleus basal; B, of _Leucosolenia_, nucleus not basal, flagellum arising from the nuclear membrane. (A, after Minchin; B, after Bidder.)] The tetraxon spicules have "equiangular" triradiate systems in the Clathrinidae, while in Leucosoleniidae they are "alate." Finally, the larva of Clathrinidae is a "parenchymula" (see p. 226), that of Leucosoleniidae an "amphiblastula." The fact that it is possible to classify the Calcarea Homocoela largely by means of histological characters is in accordance with the importance of the individual cell as opposed to the cell-layers generally throughout the Porifera, and is interesting in serving to emphasise the low grade of organisation of the Phylum. The organs of sponges are often unicellular (pores), or the products of the activity of a single cell (many skeletal elements); and even in the gastral layer, which approaches nearly to an epithelium, comparable with the epithelia of Metazoa, the component cells {187}still seem to assert their independence, the flagella not lashing in concert,[218] but each in its own time and direction. SUB-CLASS II. HETEROCOELA. [Illustration: FIG. 78.—Transverse section of the body-wall of _Sycon carteri_, showing articulate tubar skeleton, gastric ostia (_a.p_), tufts of oxeas at the distal ends of the chambers (_fl.ch_), and pores (_p_). (After Dendy.)] [Illustration: FIG. 79.—_Sycon coronatum._ At a a portion of the wall is removed, exposing the paragaster and the gastric ostia of the chambers opening into it.]