The porifera and coelentera . ither by reduction from a triaene,or, by increase of size, from a microrhabdus, derived in its turn fromreduction of an aster or a calthrop.^. The following scheme may serve to indicate the different courses ofphylogeny which are possible :— Primitive j -> euaster ^ calthrops -^ triaene Globule [ ^ ^ ^ t or Concretion J -^ streptaster ^ niicrorliabd. ^ niacrorliabd. Arrangement of the Spicides in the Skeleton.—By the arrangementof the megascleres two types of skeleton can be distinguished in theorder Tetractinellida : the irregular, seen in the Lithistida, and
The porifera and coelentera . ither by reduction from a triaene,or, by increase of size, from a microrhabdus, derived in its turn fromreduction of an aster or a calthrop.^. The following scheme may serve to indicate the different courses ofphylogeny which are possible :— Primitive j -> euaster ^ calthrops -^ triaene Globule [ ^ ^ ^ t or Concretion J -^ streptaster ^ niicrorliabd. ^ niacrorliabd. Arrangement of the Spicides in the Skeleton.—By the arrangementof the megascleres two types of skeleton can be distinguished in theorder Tetractinellida : the irregular, seen in the Lithistida, and a fovfChoristida; and the radiate type (Fig. 91), characteristic of the vastmajority of Choristida. Even in the former tyi)e, however, alltriaenes when present near the surface have the rhabdome directedtowards the centre, and to this extent exhibit a radiate structure. In most, if not all, Choristida the young sponge has a radiate structurewhen still quite small, the spicules being arranged in sheaves between 136 SPONGES. Fir,. Jlcxle of arraiij;eiiifint of spicules in a youngStollfdtid sponge, Dragmastra nornwni, Sol). (Afteriiolliis.) the incurrent folds of the canal system, with their main sliafts reachingfrom tlie centre to the periphery (Fig. 91). During subsequent growth the new spicules, which areformed after the sponge has ex-ceeded a certain size, may in afew instances be disjiosed irregu-larly, so that the full - grownsponge exhibits no trace of theradiating arrangement, exceptperhaps close to the outer sur-face ; most usually, however, thespicules formed later retain theradial arrangement, so that thespicule sheaves of the earlierstage are converted into fibresradiating from the centre to theperiphery, often with a pro-nounced spiral twist. The surface of the spongemay become hispid by theprojection of radially arrangedsjjicnles be3ond the limitingepithelium of the body wall, and the hispidating spicules may bespecially differentiated to form protecting fringes
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Keywords: ., bookcentury1900, bookdecade1900, booksubje, booksubjectctenophora
