Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/876

852 PROTOZOA [RADIOLARIA. which are totally devoid of skeleton. Similarly it does not appear to be a matter of great significance that some forms (Polycyttaria) form colonies, instead of the central capsules separating from one another after fission has occurred. It is important to note that the skeleton of silex or acanthin does not correspond to the shell of other Gymnomyxa, which appears rather to bo represented by the membranous central cap sule. The skeleton does, however, appear to correspond to the spicules of Helio/wa, and there is an undeniable affinity between such a form as Clathrulina (Fig. VII. 2) and the Sphrerid Peripylsea (such as Heliosplutra, Fig. XIV. 14). The Radiolaria are, however, a very strongly marked group, definitely separated from all other Gymnomyxa by the membranous central capsule sunk/in their proto plasm. Their differences inter sc do not atl ect their essential struc ture. The variations in the chemical 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 part of the organism so little characteristic of the group that it may be wanting altogether. It is not known how far the form-species and form-genera which have been distinguished in such profusion by 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 local and conditional varieties of a single Protean species. The same remark applies to the species discriminated among the shell-bearing Keticularia. It must not be supposed, however, that less importance is to be attached to the distinguishing and record ing of such forms because we are not able to assert that they are permanent species. The yellow cells (of spherical form, 005 to 15 of a millimetre in diameter) which occur very generally scattered in the extra- capsular protoplasm of Radiolaria were at one time regarded as essential components of the Radiolarian body. Their parasitic nature is now rendered probable by the observations of Cien- kowski (43), Brandt (44), and Geddes (45), who have established that each cell h;is a cellulose wall and a nucleus (Fig. XIV. 6 to 13), that the protoplasm is impregnated by chlorophyll which, as in Diatoms, is obscured by the yellow pigment, and that a starch- like substance is present (giving the violet reaction with iodine). Further, Cienkowski showed, not only that the yellow cells multiply by fission during the life of the liadiolarian, but that when isolated the} continue to live ; the cellulose envelope becomes softened ; the protoplasm exhibits amoeboid movements and escapes from the envelope- altogether (Fig. XIV. 13) and multiplies by fission. Brandt has given the name Zooxanthella nutricola to the parasitic unicellular Alga thus indicated. He and Geddes have shown that a similar organism infests the endoderm cells of Anthozoa and of some Siphonophora in enormous quantities, and the former has been led, it seems erroneously, to regard the chlorophyll corpuscles of Hydra viridis, Spongilla, and Ciliata as also parasitic Alga?, for which he has coined the name Zoojhlorella. The same arguments which Brandt has used to justify this view as to animal chlorophyll would warrant the creation of a genus " Phytochlorella " for the hypothetical Alga which has hitherto been described as the "chlorophyll corpuscles" of the cells of ordinary green plants. ZooMinthella nutricola does not, for some unknown reason, infest the Acanthometridea, and it is by no means so universally present in the bodies of the Silico-skeleta as was supposed before its parasitic nature was recognized. 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 pro toplasm. A feeble vibrating movement of the pseudopodia has been occasionally noticed. The production of swarm-spores has been observed only in Acanthornetra and in the Polycyttaria and Thalassicollidre, and only in the two latter groups have any detailed observations been made. Two distinct processes of swarm-spore production have been observed by Cienkowski (43), confirmed by Hertwig (46) dis tinguished by the character of the resulting spores which are called "crystalligerous" (Fig. XIV. 15) in the one case, and "di morphous" in the other (Fig. XIV. 16). In both processes the nucleated protoplasm within the central capsule breaks up by a more or less regular cell-division into small pieces, the details of the process differing a little in the two cases. In those individuals which produce crystalligerous swarm -spores, each spore encloses a small crystal (Fig. XIV. 15). On the other hand, in those indi viduals which produce dimorphous swarm-spores, the contents of the capsule (which in both instances are set free by its natural rupture) are seen to consist of individuals of two sizes " macro- spores" and "microspores," neither of which contain crystals (Fig. XIV. 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 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 (macrospores and microspores), which in a manner analagous to that observed in the Volvocinean Flagellata copulate (permanently [ UF J) with /tie another (the larger with the smaller) before proceeaiag to develop. The adults resulting from this process would, it is suggested, pro duce in their turn crystalligerous swarm-spores. Unfortunately we have no observations to support this hypothetical scheme of a life-history. 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, and an agreement, though of a negative character, with the Reticu laria. Simple fission of the central capsule of adult individuals and subsequently of the whole protoplasmic mass has been observed in several instances, and is probably a general method of reproduction in the group. The siliceous shells of the Radiolaria are found abundantly in certain rocks. They furnish, together with Diatoms and Sponge- spicules, the silica which has been segregated as flint in the Chalk formation. They are present in quantity (as much as 10 per cent.) in the Atlantic ooze, and in the celebrated "Barbados earth" (a Tertiary deposit) are the chief components. GRADE B. CORTICATA, Lankester, 1878(64). Characters. Protozoa in which the protoplasm of the cell-body, in its adult condition, is permanently differentiated into two layers, an outer denser cortical substance and an inner more fluid medul lary substance (not to be confused with the merely temporary distinction of exoplasm and endoplasm sometimes noted in Gymnomyxa, which is not structural but due to the gravitation and self-attraction of the coarser granules often embedded in the uniformly fluid protoplasm). Since the Corticata have developed from simple Gymnomyxa exhibiting both amoeboid and flagellate phases of form and activity, it results (1) that the forms of the body of many Corticata are traceable to modifications of these primitive forms ; (2) that the young stages of the Corticata are in the lower classes of that group typical flagellulffi or amcebulae ; and (3) that there are certain archaic forms included in those lower classes whose position there is doubtful, and which might be with almost equal propriety assigned to the Gymnomyxa, since they are transitional from that lower grade to the higher grade of Corticata. CLASS I. SPOROZOA, Leuckart (47) ; Syn. Grcgarinida, Auct. Characters. Corticata parasitic in almost all classes and orders of animals, imbibing nutriment from the diffusible albuminoids of their hosts and therefore mouthless. In typical cases there is hatched from a chlamydospore one or more modified nucleate or non-nucleate flagellulse (falciform young, drepanidium phase). The flagellula increases in size and differentiates cortical and medullary substance. Fission is common in the younger stages of growth. The movements now become neither vibratile nor amoe boid but definitely restrained, and are best described as " eugle- noid" (cf. Flagellata, Fig. XX. _27, 28). The nucleus is single, large, and spherical. No contractile vacuole and rarely any vacuole is present. A size of T Vth inch may be attained in this phase, which may be definitely spoken of as the euglena phase corre sponding to the amoeba phase of Gymnomyxa. It is usually of oblong form, with sac-like contractile wall of cortical substance, but may be spherical (Coccidiidea) or even amoeboid (Myxosporidia). Conjugation, followed directly or after an interval by speculation, may now ensue. The conjugated individuals (two), or sometimes a single individual, become encysted. The contents of the cysts now rapidly divide (by a process the details of which are unknown) into minute ovoid nucleated (?) bodies ; sometimes a portion of the protoplasm is not converted into spores but may form sporoducts (cf. capillitium of Mycetozoa). Each piece acquires a special chitin-like colourless coat, and is then a chlamydospore. Rarely one spore only is formed from the whole contents of a cyst. The spore-coat is usually thick, and remarkable for processes and other accessory developments. The included protoplasm of the chlamydo spore frequently divides into several pieces before hatching. These usually, when set free from the spore-coat, have the form of modified nucleated flagellulse, i.e., flagellulse in which the protoplasm is not drawn out into a thread-like flagellum but exhibits an elongate form, uniformly endowed with vibratile activity. With few (if any) excep tions, the falciform young thus characterized penetrates a cell of some tissue of its host and there undergoes the first stages of its growth (hence called Cytozoa). In some forms the pre-cystic phase never escapes from its cell host. In other cases it remains connected with the hospitable cell long after it has by growth exceeded by many hundred times the bulk of its quondam entertainer; often it loses all connexion with its cell host and is carried away to some other part of the infested animal before completing its growth and

sucystiiig.