Thuricola valvata ([Plate III]., No. 72) possesses a hinge-like process which closes up like a door when the animal contracts itself into its case. This very effectually protects it from assault. Both portions of the valve are capable of extension. Another group of ciliate infusoria also possess a limited number of cilia, but these, although restricted to the under surface of their bodies, have an unrestricted range of motion. The group are all free swimmers, belonging to the genus Oxytricha. They possess two separate alimentary orifices, neither of which are situated at the extremities or encased by a dense integument. Their locomotive organs consist either of setæ, vibratile cilia, or non-vibratile styles or uncini, variously situated, and all serving to make these infusorial animals very active ([Plate III]., Nos. 73 and 77). A typical species is the mussel-animalcule (Stylonychia, [Fig. 338]), common in all infusions and pools of water. Its body is oval and flattened, and about 1⁄100th of an inch in length. At one end a funnel-shaped depression or mouth, with a ciliated margin, leads to the inner part of the body, in which are two oval bodies, a nucleus and a contractile vacuole, which is seen to contract rhythmically. The creature can also stalk along by means of its cilia or setæ, and set up currents to the mouth. [Plate III]., Nos. 70, 71, 72, 73, and 74, are types of these interesting bodies.
Fig. 337.—Nodding-bell animalcule (Epistyles operculata) × 250 (Warne).
Fig. 338.—Mussel-animalcule (Stylonychia mytilus) under surface.
a. Mouth; b. Contractile vacuole; c. Nucleus. (Magnified 150 diameters.
Dr. Balbini believes a true sexual generation occurs among these organisms, but, with the exception of the Paramecium, this has not been seen to take place; even Gruber’s more recent investigations appear to be inconclusive on this point. Conjugation, however, it is said takes place among some attached forms, as in the Stentors. These have been seen to put forth a bud from the body base, and soon after become free swimming bodies. The trumpet-animalcule (Stentor), a conspicuous member of the ciliata, is comparatively large, being about the 1⁄25th of an inch in length when extended to the full size. It is usually found attached to the under sides of duckweed, and is continually changing its form from that of a small knob when contracted, to the trumpet shape seen in [Fig. 339], No. 6, when fully extended, and from which it derives its name. The long cilia projected from the upper part form a spiral within the margin of the open mouth leading to the digestive sac. A contractile vacuole lies to the right of the oral opening. New individuals are produced by the process of budding, and in the form of ciliated embryos from the nucleus. Stentors are commonly met with in fresh water, and are usually of a brilliant green colour. These little bodies will bear cutting up: if only a fragment of the nucleus be included in the section, the injury is soon repaired.
Rotifera, or Wheel-animalcules ([Fig. 339]).—In this group we have a higher type of animal, with a more complex organisation than those previously noticed. The great majority inhabit fresh water, and are readily developed in hay infusions, in bog-moss, in house-top gutters, everywhere if looked for after a shower of rain. The rotating organs from which these fascinating animalcula derive their name consist of two disc-like bodies whose margins are fringed with rows of cilia, which create currents toward the oral aperture, and which have given rise to the optical delusion of rotating wheels. The disposition of the cilia is so arranged as to bring food to the rotifer and conduct it to the mastax or digesting apparatus—a muscular bulb moved by a series of muscles—the gastric glands and stomach. The great transparency of the whole structure permits of the animal economy being easily studied. The body is covered with a horny envelope of two layers, and is divided into segmental divisions, which slide into each other telescopic fashion. Consequently, as the water dries up, the animal is for a long time rendered indestructible and capable of resisting varying temperatures and the action of caustic reagents.
Rotifers are oviparous, and their eggs are conspicuous and of three kinds. The common soft-shelled eggs produce females, the smaller and more spherical produce males. The ephippial, or summer eggs, are often beset with spines or bosses; these have only a membranous covering, and are hatched soon after they are laid, or before leaving the ova sac. The male rotifer is but a third of the length of the female, often without cilia, and appears to have no alimentary tract; indeed, the only internal organ is a large sperm sac. Rotifers have been divided by Dr. Hudson and the late Mr. Gosse in their charming work on these very interesting “Wheel-animalcules” into four orders, according to their powers of locomotion, as follows:—(1) Rhizota, the rooted; (2) Bdelloida, the leech-like, that swim and creep like a leech; (3) Ploïma, the sea-worthy, that only swim with their ciliary wreath; (4) Scirtopoda, the skippers, that swim with their cilia and skip with arthropodous limbs. These, again, are subdivided into families. With such hardy creatures as Philodina, Adineta, Brachionus, &c., creatures to whom extremes of cold, heat, and drought are the ordinary conditions of life, nothing can be easier to keep going throughout the year. Mr. C. F. Rousselet, who has so thoroughly succeeded in mounting Rotifers with their cilia fully extended, recently exhibited at one of the evening meetings of the Royal Microscopical Society, London, no less than four hundred specimens in a natural and perfect condition, the nervous system being seen more clearly from its successful staining throughout the body than in the living rotifer.
