Pterodína pat´ina ([Pl. XI.] fig. 3).—This species has a shell or carapace on the back, a two-lobed rotatory organ, two eyes, and a slender wrinkled tail ciliated at the extremity. The curved alimentary canal, and the two strong muscles inclined at an angle, are easily distinguishable.
Flosculária ornáta ([Pl. XI.] fig. 4) is a very beautiful member of the Rotatoria, and is found adhering to Confervæ and other water-plants. The body is club-shaped, and contained in a transparent tube, the ringed narrower portion being fixed to its base. The rotatory organ is divided into five or six lobes, furnished with long, slender, radiating tentacular filaments; these are not vibratile like ordinary cilia, but can be slowly moved. In the contracted state, the filaments form a pencil-like bundle.
Examination, &c.—The Rotatoria are best examined in the living state, the drop of water in which they are viewed being very small, so that their movements may be impeded; and while they are struggling to escape, the various parts of the body will come into view. Their preservation has been attempted by drying on a slide; but when dead they become so contracted and altered, that it is difficult to make out their structure. Should the observer wish to record any observations on their reproduction or habits, it will be well to preserve a specimen of the jaws and teeth, as the species might be with certainty identified by careful examination of their minute structure.
Entozóa (ἑντὀς, within, ξῶον, animal).—This class consists of the parasitic worms, as the Tape-worm (Tænia), the Thread-worm and Round-worm (As´caris), which live within the bodies of man and animals. It also includes the microscopic eel-like animalcules (species of Anguil´lula) which are found in sour paste (A. glútinis), in vinegar (A. acéti), and in blighted wheat (A. trit´ici). Some of the species of allied genera are met with in damp moss and in the débris or fragments of vegetable substances decaying in water. The general appearance of the microscopic species is that of a minute colourless eel, writhing in the water ([Pl. XI.] fig. 1). Their internal organs are difficult to distinguish. The alimentary canal is usually evident, and dilated into a kind of stomach, containing near its commencement some rod-like or otherwise-formed teeth. In the species figured there are two apparently tubular lancets, which are capable of protrusion, and evidently serve to wound the prey.
CHAPTER XII.
RADIATA.
DESCENDING in the scale of animal organization, we come next to the subkingdom Radiáta, or that in which the parts are arranged in a radiate manner around a centre. Of this there are three classes,—the Echinoder´mata (ἑχῖνος, hedgehog, δἑρμα, skin), containing the Sea-urchins (Echínus), Starfishes, &c., in which the skin is furnished with hard calcareous projecting spines or curiously formed imbedded calcareous corpuscles, forming a rudimentary skeleton; the Acaléphæ (ἁκαλἡφη, a nettle), or Sea-nettles; and the Pol´ypi (πολὑς, many, ποῦς, foot), to which we shall confine our notice. It may be remarked that the last two classes have recently been united to form the single class Cœlenteráta (κοῖλον, hollow, ἔντερον, intestine).
Polypi.—These animals are mostly marine. They are either single (Pl. XI. fig. 5), or compound ([Pl. XI.] fig. 15), i. e. the bodies are united; in the latter case the bodies being usually situated in horny cells upon a branched polypidom. But in many of them, which do not occur in this country, there is an internal solid calcareous skeleton, of which coral is an example. The animal bodies are soft, and furnished at the front end with a crown of tentacles (fig. 15 a); these are contractile, and serve to enable the animals to catch their prey. The horny, branched, and plant-like polypidoms are often found on the seashore, and are popularly confounded with sea-weeds.
Hy´dra vulgáris ([Pl. XI.] fig. 5) is a fresh-water species, which is commonly met with among collections of water-plants, and may generally be obtained by collecting some of these and placing them in a glass jar of fresh water. When the water has stood for some hours, the Polypes will be seen, on careful examination, adhering to the sides of the glass. The body of the animals is cylindrical, hollow, and furnished with from six to ten tentacles, arranged in a circle, in the centre of which is the mouth. The tentacles are hollow, and communicate with the cavity of the body. On examination with a high power, the tentacles will be found to exhibit minute oval sacs, containing a long fibre coiled up within them; and when the tentacles are touched by any foreign body, the fibres are suddenly discharged. These are the stinging or urticating organs. The Hydræ move very slowly; but the body is very contractile, and is often seen of various forms. When a minute animal, as an Entomostracan, happens to come into contact with the tentacles, these curve around it, holding it firmly, and finally bringing it to the mouth. It is then forced into the cavity of the body of the animal, where it is digested, the remains being discharged at the mouth. The movements of the Hydra, when devouring its prey, form a very curious and interesting spectacle. The Hydræ are propagated by budding or gemmation, also by the formation of capsules in the walls of the body, containing ova and spermatozoa. The young Polypes formed by budding are represented in the figure, adhering to the base of the parent.
Sertulária púmila ([Pl. XI.] fig. 15) is a marine species, the polypidom being frequently found adhering to Fuci and other sea-weeds; it is about half an inch long. The cells are opposite, pointed at the ends, and with an oblique orifice. The tentacles are fourteen in this species. In the summer large ovate cells are found, arising from the polypidom; these contain the eggs, and are called ovisacs or ovig´erous vesicles.