The porifera and coelentera . cule with threemorphological axes has typically six rays, but the number of thelatter may be reduced to two or three or even to a single one. The number of axes which can be recognised in a giventype of spicule is expressed by adjectives terminating in axon, 38 SPONGES combined with a Greek numeral, as monaxon, triaxon, number of rays present, on the other hand, is connoted in asimilar manner by substantives terminating in actine, or by-adjectives terminating in actinal,—for example, diactine, ordiactinal spicule. The former series of terms is usually em-p
The porifera and coelentera . cule with threemorphological axes has typically six rays, but the number of thelatter may be reduced to two or three or even to a single one. The number of axes which can be recognised in a giventype of spicule is expressed by adjectives terminating in axon, 38 SPONGES combined with a Greek numeral, as monaxon, triaxon, number of rays present, on the other hand, is connoted in asimilar manner by substantives terminating in actine, or by-adjectives terminating in actinal,—for example, diactine, ordiactinal spicule. The former series of terms is usually em-ployed to express rather the ideal type of any given spicule, thelatter to describe its actual condition. The following types of spicule can be recognised in spongesgenerally, each type exhibiting in its turn innumerable variations :— (1) The monaxon type of spicule, built upon a single axis, andhaving therefore simply the form of a rod or needle (Fig. 47, a andh). A monaxon spicule may be either monactinal (Fig. 47, h) or. Fig. 47. Types of spicules (inegascleres). a, rhalxlus (diactinal monaxon); h, stylus (monactinalmonaxon); c, triactine ; ; /, desma ol an aiionKJClailiiieLithistid (secondarily polyaxon) ; g, sterraster (polyaxoii); h, radial section thro\i>:h tin outer?part of 3, showinj; two actines soldered together by intervening silica, the free ends terminaiin-in recurved spines, and the axis traversed by a central fibre. diactinal (Fig. 47, a\ the two rays in the latter case being placedin the same straight line. The axis may be straight or curved(Fig. 48, (/, h, c, etc.). (2) The triaxon type, ciiaracteristic of Hexactinellids (Fig. 47,).The primitive spicule is laid down along three axes which cut oneanother at right angles at a central point, producing a six-rayed orhexactinal spicule, which may undergo a secondary reduction of therays ; but so long as more than one ray persists, it meets its fellowor fellows at angles of 90 or 180.
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Keywords: ., bookcentury1900, bookdecade1900, booksubje, booksubjectctenophora
