318 



ROTIFERA, OB WHEEL-ANIMALCULES. 



layers of which are extremely thin and flexible in some species ; 

 whilst in others the outer one seems to possess a horny consist- 

 ence, and may even contain siliceous or flinty matter, like the 

 sheaths of the lower Infusoria. In the former case, the whole 

 integument is drawn together in a wrinkled manner, when the 

 body is shortened ; in the latter, the sheath acts as a kind of 

 cell, into which the head and ciliary apparatus can be completely 

 retracted. These last would 

 seem to have some relation with 

 the Polypifera ; and we shall 

 hereafter see that the affinity 

 may be regarded as very close. 

 On the other hand, the former 

 present an external resemblance 

 to the Yermiform tribes ; which, 

 also, will be found to increase on 

 a closer examination : and there 

 are also species, of which the 

 sheath bears a strong likeness to 

 that of some Entomostracous 

 Crustacea ( 807). Hence we 

 may regard this group as con- 

 necting the Articulated classes 

 with the Zoophytes. 



85 7. As a characteristic illus- 

 tration of the class, we shall select 

 the common "Wheel- Animalcule, 

 Rotifer vulgaris. The body, 

 when extended, possesses consi- 

 derable length in proportion to wh eels folded up and drawn in ; a, the 

 . ,. . . head with the eye-spots ; 6, the wheels ; 



ItS diameter, and has much Of the c, water-siphon ; d, masticating appara- 



Vermiform Or Worm-Hkp a^nprt tus; ' ""vary glands; //> intestinal 

 m-llKe aspect , canal . ^ its dilated termination . hi glan . 



this is increased When a Slight dular apparatus surrounding it; i, young 



ones nearly complete ; k, eggs ; I, tail. 



contraction draws the external 



membrane, which is here thin and very flexible, into transverse 

 wrinkles, that seem to indicate the segments of the trunk. The 

 posterior extremity is prolonged into a tail, possessing three 



FIG. .537 WHEEL ANIMALCULES ; A, 

 with the wheels expanded ; B, with the 



