We have called the masticatory apparatus of the Rotifers a _gizzard_; but Mr. Gosse, who has done most to elucidate its structure, contends that it is a _mouth_; and in some species it is frequently protruded, and used like the mouth of higher animals. Taking one of the most typical forms of this organ, and drawing our illustrations from Mr. Gosse's admirable paper in the "Transactions of the Royal Society," we may describe it, when completely developed, as consisting of three lobes, having a more or less rounded form. The eminent naturalist we have named calls the whole organ the _mastax_, and states that it is composed of dense muscular fibre. The tube which leads down to it he designates the "buccal (mouth) funnel," and the tube that issues from it, and conveys the food to the digestive sac or stomach, he calls the _oesophagus_, in conformity with the nomenclature applied to creatures whose mouths are in the usual place. Inside the mouth-gizzard are placed two organs, which work like hammers, and which Mr. Gosse therefore names _mallei_. The hammers work against a sort of anvil, which is called _incus_, the Latin for that implement. Each hammer consists of two portions articulated by a hinge joint. The lower portion, the _manubrium_, or handle, gives motion to the upper portion, which from its shape is named the _uncus_, or hook. The _unci_ are furnished with finger-like processes of teeth, which vary in number. There are five or six in the best developed specimens. These hooks or teeth work against each other, and against the _incus_, or anvil, which consists of distinct articulated portions, of which the principal are two _rami_, or branches, jointed so that they can open and close like a pair of shears. These two rest upon the third portion, which is called the _fulcrum_. Some faint idea of the working of the toothed hammers may be obtained by rubbing the knuckles of both hands together, but the motion is more complicated, and the _rami_ play their part in the trituration of the food. Mr. Gosse states that when an objectionable morsel has got as far as this mouth-gizzard, "it is thrown back by a peculiar scoop-like action of the _unci_, very curious to witness." The foregoing diagram will help the reader to comprehend this description, but no opportunity should be lost for viewing this remarkable organ busy at work in the living animals. [Illustration: Gizzard of Notomata.] The respiration of the Rotifers is supposed to be effected by the passage of water through vessels running round them, and called the "water vascular system," and in addition to their eyes, which often disappear in adult specimens, the organ we described as standing out like a pig-tail, as our acquaintance crawled along, is thought to act as an _antenna_, or feeler, and brings its possessor in further relation to the external world. It is also called the _calcar_, or spur, and is furnished with cilia or bristles at its extremity.