. A manual of zoology. Zoology. SILICISPONGI/E 205 glass sponges)—have three crossed axes (six threads radiating from a common point)—hence Hexactinellida;. The mesoderm is scanty and in consequence the canals are loose-meshed, lacunar spaces and the ampullae large and barrel- formed. In the Tetraxonia the mesoderm is usually abundant and the canal system well developed. The four-axial spicules of the Tetractinellidae must be regarded as the fundamental skeletal type. From this are derived the compact frameworks of the Lithislids and the monaxial spicules of the Monactinellid;e. In both groups
. A manual of zoology. Zoology. SILICISPONGI/E 205 glass sponges)—have three crossed axes (six threads radiating from a common point)—hence Hexactinellida;. The mesoderm is scanty and in consequence the canals are loose-meshed, lacunar spaces and the ampullae large and barrel- formed. In the Tetraxonia the mesoderm is usually abundant and the canal system well developed. The four-axial spicules of the Tetractinellidae must be regarded as the fundamental skeletal type. From this are derived the compact frameworks of the Lithislids and the monaxial spicules of the Monactinellid;e. In both groups the spicules may be united by secondary deposits of silica to an extensive framework; or the union is affected by spongin, which, if the spicules disappear, forms the whole skeleton (horny sponges); or, as in slime-sponges, the whole skeleton may be lost. Sub Order I. TRIAXONIA. HexactinelliDjE, chiefly deep seas; Euplecidla aspergUlum, Venus' fiower-basket. Hyalonema. Sub Order II. TETRAXONIA. Typical are the largely extinct Lithistid^ (some genera— Discodennia—persist in deep seas) and Tehl^ctinellid^: Geodia* Near here apparently belongs OscarcUa* without a skeleton (Myxospongia). Mon- ;, spiculcs united by spongin (Cornacuspongia); can even be entirely replaced by that substance. Numerous marine forms, and the fresh- water (Spongilla* Epkydalia*), usually colored green by algae. They are distinguished by formation of gemmulce or statohlasts. At times the. Fig. 167.—Skeletal structures of sponges (after Schulze and Maas). i, Horn fibre of bath sponge with spongioblasts; 2-7, spicules of, 2, Esperia; 3, 4, Corticum; 5, Mysilla; 6, Telhya; 7, Farrea. protoplasm divides into round bodies, as large as the head of a pin and these become surrounded by a firm membrane often strengthened by collar-button-like spicules, the amphidiscs. These statoblasts survive times of freezing or drought. On return of good conditions the contents escape and form
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
