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 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.
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.)
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.
The Heterocoela present a series of forms of successive grades of complexity, all derivable from the Ascons, from which they differ in having a discontinuous gastral layer. The simplest Heterocoela are included in the family Sycettidae, of which the British representative is Sycon (Fig. 79). In Sycon numerous tubular flagellated chambers are arranged radially round a central cavity, the "paragaster," into which they open (Figs. 78, 79). The chambers, which are here often called radial tubes, are close set, leaving more or less quadrangular tubular spaces, the inhalant canals, between them; and where the walls of adjacent chambers come in contact, fusion may take place. Pores guarded by porocytes put the inhalant canals into communication with the flagellated chambers. The paragaster is lined by pinacocytes; choanocytes are confined to the flagellated chambers.
The skeleton is partly defensive, partly supporting; one set of spicules strengthens the walls of the radial tubes and forms collectively the "tubar skeleton." It is characteristic of Sycettidae that the tubar skeleton is of the type known as "articulate"—i.e. it is formed of a number of successive rings of spicules, instead of consisting of a single ring of large spicules which run the whole length of the tube.