Text Appearing Before Image: 2 24 THE AMERICAN MUSEUM JOURNAL choanocytes and their digestible parts absorbed. What is left is discarded and flows with the current out through the osculum at the summit of the vase. The mesoderm (mes.) is a thin jelly-like layer between the ectoderm and endoderm. It contains scattered amoeboid cells and the reproductive elements, and is the origin of the skeleton. 2. The Canal Systems. In the form of sponge just described the mesoderm is extremely thin, but if, as in the majority of sponges, there is a greater or less thickening of this layer, the pores will no longer be perforations, but will become transformed into tubes or canals Text Appearing After Image: FIG. 9.—A SYCON SPONGE (Graiitia ciliata FlemingXoROWINQ ON A MUSSEL SHELL Star-shaped colonies of the Ascidian Botrylliis are also growing on the same shell. (See p. 221, Fig. 3), which may branch and be modified in various ways. This gives rise to three general types of sponges which are therefore based mainly on the arrangement and varia- tions of the pore- and canal-systems. These are known as (a) The Ascon Type, (b) The Sycon Type, Cc) The Rhagon Type.
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