MICROSCOPIC FORMS OF ANIMAL LIFE. 55 



body is shortened ; in some of the latter the sheath has the form of a 

 polype-cell, and the body lies loosely in it, the inner layer of the integu- 

 ment being separated from the outer by a considerable space (Fig. 312); 

 whilst in others the envelope or lorfca is tightly fitted to the body, and 

 strongly resembles the horny casing of an Insect or the shell of a Crab, 

 except that it is not jointed, and does not extend over the head and tail, 

 which can be projected from the openings at its extremities, or com- 

 pletely drawn within it for protection (Fig. 313). In those Kotifera in 

 which the flexibility of the body is not interfered with by the consolida- 

 tion of the external integument, we usually find it capable of great varia- 

 tion in shape, the elongated form being occasionally exchanged for an 

 almost globular one, as is seen especially when the animals are suffering 

 from deficiency of water; whilst by alternating movements of contraction 

 and extension, they can make their way over solid surfaces, after the 

 manner of a Worm or a Leech, with considerable activity, some even of 

 the loncated species being rendered capable of this kind of progression 

 by the contractility of the head and tail. All these, too, can swim 

 readily through the water by the action of their cilia ; and there are 

 some species which are limited to the latter mode of progression. The 

 greater number have an organ of attachment at the posterior extremity 

 of the body, which is usually prolonged into a tail, by which they can 

 affix themselves to any solid object; and this is their ordinary position, 

 when keeping their ' wheels' in action for a supply of food or of water; 

 they have no difficulty, however, in letting-go their hold and moving 

 through the water in search of a new attachment, and may therefore be 

 considered as perfectly free. The sessile species, in their adult stage, on 

 the other hand, remain attached by the posterior extremity to the spot 

 on which they have at first fixed themselves ; and their cilia are conse- 

 quently employed for no other purpose than that of creating currents in 

 the surrounding water. 



447. In considering the internal structure of Rotifera, we shall take 

 as its type the arrangement which it presents in the Rotifer vulgaris 

 (Fig. 310); and specify the principal variations exhibited elsewhere. 

 The body of this animal, when fully extended, possesses greater length 

 in proportion to its diameter than that of most others of its class; and 

 the tail is composed of three joints or segments, which are capable of 

 being drawn up, one within another, like the sliding tubes of a telescope, 

 each having a pair of prongs or -points at its extremity. Within the ex- 

 ternal integument of the body are seen a set of longitudinal muscular 

 bands (Ji), which serve to draw the two extremities towards each other; 

 and these are crossed by a set of transverse annular bands, which also are 

 probably muscular, and serve to diminish the diameter of the body, and 

 thus to increase its length. Between the wheels is a prominence bearing 

 two red spots (b), and having the mouth (a) at its extremity; these red 

 spots differ altogether from those common in Infusoria and Protophyta, 

 each having a minute highly-refracting spherical lens set in red pigment, 

 and being clearly a rudimentary eye; and the prominence that bears them 

 may be considered, therefore, as a true head, notwithstanding that it is 

 not clearly distinguishable from the body. This head also bears upon its 

 under surface a projecting spur-like organ (d), which was thought by 

 Prof. Ehrenberg to be a siphon for the admission of water to the cavity 

 of the body for the purpose of respiration; this, however, is certainly not 

 the case, the 'spur' being imperf orate at its extremity; and there seems 

 much more probability in the idea of Dujardin, that it represents the 



