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a tail, inasmuch as its position is anterior to the cloacal orifice. A 

 minute projection behind the cloaca forms a sort of tail. When not 

 swimming or rotating, the head assumes a rounded outline (PI. XIII. 

 fig. C), displaying through the transparent integument an oval mark on 

 each side, within which a tremulous motion is perceived ; but at the 

 pleasure of the animal a semi-globular lobe is suddenly projected from 

 each of these spots by evolution of the integument. These projec- 

 tions have suggested the trivial name of aurita. Each lobe is 

 crowned with a wheel of cilia, the rapid rotation of whose waves 

 forms the principal source of swift progression in swimming. The 

 protrusions of these lobes are evidently eversions of the skin, ordina- 

 rily concealed in two lateral cavities. They may be protruded by 

 pressure, and are then seen to be covered with long but firm and 

 close-set cilia, which are bent backward and moved more languidly as 

 death approaches. The whole front is also fringed with short, vibratile 

 cilia, which extend all along the face, as far as the constriction of the 

 neck. The whole body is clear and nearly colourless, but its trans- 

 parency is much hindered by the net-work of dim lines and corruga- 

 tions, that are everywhere seen, particularly all about the head. It 

 is only by subjecting the animals to various degrees of pressure by 

 means of a compressorium, or of glass plates, and by the examina- 

 tion of a great number of specimens, that a satisfactory demonstra- 

 tion of the internal structure can be obtained. 



The digestive apparatus shall have our first attention. Near the 

 head is seen a large oval or cylindrical transparent sac, placed to- 

 wards the ventral rather than the dorsal side, but capable of much 

 motion and change of place ; sometimes it is suddenly drawn down 

 nearly to the foot, while at others its front part is brought into con- 

 tact with the matters on which the animal feeds. This viscus I shall 

 call the gizzard, as its office is the trituration of the food to prepare 

 it for digestion. 



The implements of mastication within the gizzard are elaborate and 

 complex. They consist of two bent jaws, which work upon a central 

 table, and have been likened to a pair of hammers working on an anvil ; 

 I shall therefore name the working arms the mallei, and the central 

 table the incus. This apparatus, with many modifications of detail, 

 characterizes the dental system of a large number of the Rotifera. 



When viewed in situ, the incus is seen to consist of a long slender 

 foot, apparently dilated at the bottom, and carrying on its upper end 

 a complex array of tabular pieces, which, from the obscurity of this 

 part, are very difficult to be made out. They appear, however, to 



