Fig. 332.—Acineta, magnified 600 diameters (Warne).
Among those widespread, and in some respects heterogeneous, forms of life associated under the comprehensive title of Infusoria, we encounter types that not only differ very widely from one another, but which occupy a different rank or position, so to speak, with regard to the relation they bear to each other, and also to the outlying representatives of the series—differences that permeate throughout the ranks of this extensive group. Furthermore, a considerable number of Infusorial animalcules foreshadow or typify, in a corresponding degree, the separate or associated cell elements out of which higher tissue structures—metazoic organisms—are built up. We may take the well-known example Euglena viridis ([Plate III]., No. 67), or Paramecium (No. 74), and their allies; these would appear to be the prototypes of Turbellaria. Another more lowly organised group of the Ciliata exhibits a distinct and highly-interesting affinity to the Opalinidæ. There are many other species (Acineta, [Plate III]., No. 68, for instance), which at first sight would seem to stand by themselves and present no marked agreement with any metazoic type. Indeed, the function of these and other polypites consists simply in seizing food and conveying it through perforations at the extremity of each separate tentaculum to its interior. In Acineta certain of the tentacles only are suctorial, and these, being the inner ones, fulfil the ingestive function, while the peripheral series are prehensile. This stalked club-shaped body ([Fig. 332]), which fixes itself to seaweeds or Bryozoa, is seen to have a nucleus, and also clear vesicles in the body-substance; its embryos are ciliated. It is an object of considerable interest even among curious marine animalcules; one or two species inhabit fresh water. The spiral-mouthed Spirostomum are among the largest of the class, and in sunlight are visible to the naked eye as slender golden threads of about 1⁄10th of an inch in length. The mouth slit, extending half the length of the body, is bordered on one side by cilia. The body is cylindrical and the surface covered with rows of cilia. Its multiplication takes place by transverse fission through the middle.
Flagellate Infusoria.—The characteristic of this group, as its name implies, is the possession of one or more flagella or whip-like appendages, at the base of which is an opening in the denser surface layer of protoplasm, and in the interior a nucleus and one or more contractile vacuoles, and not infrequently a brilliant red spot of pigment known to microscopists as the eye-spot. The Monads, which constitute the simplest members of the group, are commonly found in fresh-water pools and vegetable infusions. The typical form consists simply of a spherical or oval cell provided with a flagellum. The Volvox was formerly placed in this group, but as it contains chlorophyll it is properly claimed by the botanist. The collared group possesses cup-like collars, and these frequently secrete horny receptacles or cups, and form elegant tree-like colonies.
The mail-coated group are of very varied form, the body being often prolonged into spiny processes. They have two long flagella which fit into grooves purposely provided. But the most interesting and remarkable are the phosphorescent animalcules (Noctiluca), whose beautiful bluish-green luminosity on the surface of the sea has attracted attention from very early periods. It was, however, not until the first half of the present century that the luminosity was discovered to be due to the presence of multitudes of these minute jelly-like spheres.
Fig. 333.—Noctiluca miliaris; magnified 150 diameters.
Fig. 334.—Pyrocystis; magnified 150 diameters.
The body of the Noctiluca ([Fig. 333]) is a nearly globular-shaped cyst, enclosed in a tough membranous wall, from a grooved opening in which a striated muscular flagellum or proboscis is projected forth, and it is by means of this the animal swims away even in rough seas. A fine whip-like flagellum is also located in the same groove. At the apex of the funnel there is a mass of protoplasm which extends itself as a widely-meshed, highly-vacuolated network to the inner wall of the cyst, whence it is believed the phosphorescent light emanates. It multiplies by self-division, first becoming encysted after withdrawing its flagellum, and then breaking up into numerous ciliated helmet-shaped swarm spores. Frequently two organisms fuse into one and then divide into spores.