THE WORM-LIKE ANIMALS 



from the circumstance that the circlets of hair situated on the head give rise, when 

 waving in the water, to the appearance' of revolving wheels. The head end of the 

 body is usually broader than the opposite extremity, and terminates in the wheel, 

 or trochal disc, the edges of which are variously lobed, and clothed with the 

 vibratile cilia, or threads. The body, which is indistinctly segmented, is either 

 naked or inclosed in a hard transparent case, or lorica, open at both ends, which 

 may be variously sculptured, and armed in front and behind with spiny processes, 

 as shown in the annexed engraving. The posterior end of the body, termed the 

 foot, ends usually in a pair of movable processes, by means of which the rotifers 

 anchor themselves to foreign bodies of various kinds. The mouth, situated in the 

 middle or at the side of the wheel disc, is a funnel-shaped cavity, leading into a 

 muscular gullet (#), provided with a peculiar armature of teeth, which serve to 

 masticate particles of food that are swept into the mouth by the movements of the 

 cilia on the wheel disc. The nervous system consists of a single large ganglion, 



situated on one side beneath the disc, 

 and sending forth nerves to the sur- 

 rounding parts, and sometimes being 

 furnished with one or more eye spots. 

 In all cases the males are smaller than 

 the females, and further differ in hav- 

 ing the alimentary canal aborted and 

 reduced to a solid chord. Wheel ani- 

 malcules are divisible into four orders. 

 Of these, the Ploima may be considered 

 the typical order of the class, on ac- 

 count of the numbers of genera, the 

 abundance of species, and the restless 

 energy, perfection of structure, and 

 superior intelligence of its members. 

 Ivocomotion is effected by means of 

 swimming with the ciliary wreath. 

 The order is divided into two sec- 

 tions, the L,oricata, which, as in 

 Noteus, represented in the annexed 

 illustration, are protected by an inclos- 

 ing shell, and the Illoricata, which, as 

 exemplified by Notommata, have the 

 integument flexible, and the body not 

 protected in a shelly case. The habits 

 of the group show considerable varia- 

 tion. Many species may be found 

 swimming freely or attached to water- 

 weeds in almost any pond, stream, or 



FOUR-HORNED ROTIFER, Noteus quadricomus. stagnant ditch; and others, like Brachi- 

 (Enlarged 300 times.) onus, one of the I^oricata, may be seen 



