. A manual of zoology. 84 MANUAL OF ZOOLOGY Sycon belongs to a type of sponges intermediate between the very simplest forms on the one hand, and the more complex on the other. The simplest and most primitive of known sponges is one named Ascetta primordialis (Fig. 37). It is vase-shaped, contracted at the base to form a sort of stalk, by the expanded extremity of which it is attached; at the oppo- site or free end is the circular osculum. So far there is a considerable resemblance to Sycon gelatinositm; but the structure of the wall in Ascetta is extremely simple. Regu- larly arranged over the
. A manual of zoology. 84 MANUAL OF ZOOLOGY Sycon belongs to a type of sponges intermediate between the very simplest forms on the one hand, and the more complex on the other. The simplest and most primitive of known sponges is one named Ascetta primordialis (Fig. 37). It is vase-shaped, contracted at the base to form a sort of stalk, by the expanded extremity of which it is attached; at the oppo- site or free end is the circular osculum. So far there is a considerable resemblance to Sycon gelatinositm; but the structure of the wall in Ascetta is extremely simple. Regu- larly arranged over the sur- face are a number of small rounded apertures, the in- halant or incurrent pores ; but, since the wall of the sponge is very thin, these apertures lead directly into the central or paragastric cavity, the long passages or canals through which the communication is effected in Sycon being absent. The wall consists of the same three layers as in Sycon ; but the middle one, though it contains a small number of spicules, is very thin ; the endoderm, which lines the. Fig. 37. — Ascetta primordialis. A portion of the wall of the vase-like sponge removed to show the para- gastric cavity. (After Haeckel.). 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 Parker, T. Jeffery (Thomas Jeffery), 1850-1897; Haswell, William A. (William Aitcheson), 1854-1925. New York, The Macmillan Company; London, Macmillan & Co. ,Ltd.
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