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RADIOLARIA
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RADIOLARIA . " The occasional See also:total See also:absence of any siliceous or acanthinous See also:skeleton does not appear to be a See also:matter of classificatory importance, since skeletal elements occur in See also:close See also:allies of those very few forms which are totally devoid of skeleton. Similarly it does not appear to be a matter of See also:great significance that some forms (Polycyttaria) See also:form colonies, instead of the central capsules separating from one another after fission has occurred. " It is important to See also:note that the skeleton of silex or acanthin does not correspond to the See also:shell of other See also:Sarcodina, which appears rather to be represented by the membranous central See also:capsule. The . skeleton does, however, appear to correspond to the spicules of See also:Heliozoa, and there is an undeniable See also:affinity between such a form as Clathrulina .and the Sphaerid Peripylaea (such as Heliosphaera, fig. III. 14). The Radiolaria are, however, a very strongly marked See also:group, definitely separated from all other Sarcodina by the membranous central capsule sunk in their See also:protoplasm. Their See also:differences inter se do not affect their essential structure. The See also:variations in the chemical See also:composition of the skeleton and in the perforation of the capsule do not appear superficially. The most obvious features in which they differ from one another relate to the form and complexity of the skeleton, a See also:part of the organism so little characteristic of the group that it may be wanting altogether. It is not known how far the form-See also:species and form-genera which have been distinguished in such profusion by See also:Haeckel as the result of a study of the skeletons are permanent (i.e. relatively permanent) physiological species. There is no doubt that very many are See also:local and conditional varieties, or even merely stages of growth, of a single Protean species. The same remark applies to the species discriminated among the shell-bearing Reticularia. It must not be supposed, however, that less importance is to be attached to the distinguishing and recording of such forms because we are not able to assert that they are permanent species. ' The streaming of the granules of the protoplasm has been observed in the pseudopodia of Radiolaria as in those of Heliozoa and Reticularia; it has also been seen in the deeper protoplasm; and granules have been definitely seen to pass through the pores of the central capsule from the intracapsular to the extracapsular protoplasm. A feeble vibrating See also:movement of the pseudopodia has been occasionally noticed. " The See also:production of swarm-spores has been observed only in Acanthometra and in the Polycyttaria and Thalassicollidae, and only in the two latter See also:groups have any detailed observations been made. Two distinct processes of swarm-spore production have been observed by Cienkowski, confirmed by Hertwig,—distinguished by the See also:character of the resulting spores, which are called ' crystalligerous ' and ' isospores ' (fig. III. 15) in the one See also:case, and ' dimorphous ' or ' anisospores ' in the other (fig. III. 16). In both processes the nucleated protoplasm within the central capsule breaks up by a more or less See also:regular See also:cell-See also:division into small Acanthometridea. pieces, the details of the See also:process differing a little in the two cases. In those individuals which produce crystalligerous swarm-spores, each spore encloses a small crystal (fig. I I I. 15). On the other See also:hand, in those individuals which produce dimorphous swarm-spores, the contents of the capsule (which in both instances are set See also:free by its natural rupture) are seen to consist of individuals of two sizes, ' megaspores ' and ' microspores,' neither of which contain crystals (fig. 111. 16). The further development of the spores has not been observed in either case. Both processes have been observed in the same species, and it is suggested that there is an See also:alternation of sexual and asexual generations, the crystalligerous spores developing directly into adults, which in their turn produce in their central capsules dimorphous swarm-spores (megaspores and microspores), which in a manner analogous to that observgd in the Volvocinean See also:Flagellata copulate (permanently fuse) with one another (the larger with the smaller) before proceeding to develop. The adults resulting from this process would, it is suggested, produce in their turn crystalligerous swarm-spores. Unfortunately we have no observations to support this hypothetical See also:scheme of a See also:life-See also:history. " See also:Fusion or conjugation of adult Radiolaria, whether preliminary to swarm-spore-production or independently of it, has not been observed—this affording a distinction between them and Heliozoa. " See also:Simple fission of the central capsule of adult individuals, pre-ceded of course by nuclear fission, and subsequently of the whole protoplasmic See also:mass, has been observed in several genera of Acantharia and Phaeodaria, and is probably a See also:general method of See also:reproduction in the group. In Spumellaria it gives rise to colonial ' Polycyttarian' forms when the extracapsular protoplasm does not See also:divide. " The siliceous shells of the Radiolaria are found abundantly in certain rocks from Palaeozoic times onwards. They furnish, together with Diatoms and Sponge spicules, the See also:silica which has been segregated as See also:flint in the See also:Chalk formation. Additional information and CommentsThere are no comments yet for this article.
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