_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).