. Elementary text-book of zoology. io8 PORIFERA. In the phylum there can be discerned at least four different types of sponges according to the distribution of the choanocytes. (i) Ascon type. In this simplest type the whole para- gastric cavity is lined by choanocytes and there are no radial canals (Fig. 39). (2) Sycon type. The choanocytes are restricted to the exhalent radial canals; inhalent canals and prosopyles are present (Fig. 42). (3) Leucon type. The choanocytes are restricted to a number of secondary radial canals opening into the primary radial canals (Fig. 43). Fig. 43.—Transverse
. Elementary text-book of zoology. io8 PORIFERA. In the phylum there can be discerned at least four different types of sponges according to the distribution of the choanocytes. (i) Ascon type. In this simplest type the whole para- gastric cavity is lined by choanocytes and there are no radial canals (Fig. 39). (2) Sycon type. The choanocytes are restricted to the exhalent radial canals; inhalent canals and prosopyles are present (Fig. 42). (3) Leucon type. The choanocytes are restricted to a number of secondary radial canals opening into the primary radial canals (Fig. 43). Fig. 43.—Transverse Sfxtion of Part of the Walt. OF A Leucon. (Diagrammatic). Paragaslric Exhalent Canal. Pore. 'Secondary In- halent Canal. Secondary Exhal- ent Canal, with Choanocytes. (4) Rhagon type. The secondary canals are contracted into small round chambers and only open into the primary radial canal by a number of exhalent canals or apopyles (Fig. 44)-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Masterman, Arthur Thomas. Edinburgh, E. & S. Livingstone
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