STRUCTURE OF WHEEL-ANIMALCULE. 



319 



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 vermiform 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 ( 882). Hence we 

 may regard this group as con- 

 necting the Articulated classes 

 with the Zoophytes. This re- 

 semblance is heightened by the 

 fact that many of these little 

 creatures are sessile or attached 

 to one spot during life, and most 

 of these envelope themselves with 

 a gelatinous case, from the top of 



Which the upper part of the body c > water-siphon ; d, masticating appara- 



J tus; , salivary glands; //, intestinal 

 With the rotatory Organs IS pro- canal ;g, its dilated termination; ft, glan- 



truded. 



FIG. 637 WHKKL AMMALCULKH ; A, 

 with the wheels expanded ; B, with the 



dulnr apparatus surrounding it ; i, young 

 ones nearly complete ; A-. ecrgs; I. tail. 



929. As a characteristic illus- 

 tration of the class, we shall select the common Wheel- Animal- 

 cule, Rotifer vulgaris. The body, when extended, possesses 

 considerable length in proportion to its diameter, and has much 

 of the Vermiform or worm-like aspect ; this is increased when a 

 slight contraction draws the external membrane, which is here 

 thin and very flexible, into transverse wrinkles, that seem to in- 

 dicate the segments of the trunk. The posterior extremity is 



