. The Cambridge natural history. Zoology. i86 PORTFERA 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/ 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
. The Cambridge natural history. Zoology. i86 PORTFERA 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/ 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. a whole capable. --n Fig. 77.—The two types of Asconid collar cells. A, of ClatliHna^ nucleus basal; B, of Le^ico- solenia, nucleus not basal, flagellum arising from the nuclear membrane. (A, after Miuchin ; B, after Bidder.) The layer of pinacocytes as is highly contractile, and is of diminishing the size of the sponge to such an extent as quite to obliterate temporarily the gastral cavity.^ The choanocytes show certain con- stant differences in structure in the families Glathrinidae and Leucoso- leniidae 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). The tetraxou spicules have " equiangular " triradiate systems in the Glathrinidae, while in Leucosoleniidae they are "; Finally, the larva of Glathrinidae is a "parenchymula" (see p. 226), that of Leucosoleniidae an " ; 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 unicellula
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