CLASS ANNELIDA. 



23 



near tlie head, and carrying upon 

 of a brightly-coloured spot. 



Wheel-animalcules, such as here 

 described, may be readily detected 

 in most ponds, ditches, or slow- 

 running streams where water- 

 plants grow abundantly. Though 

 microscopic in their dimensions, 

 they are interesting objects of 

 study, owing to the facility with 

 which their transparent skin allows 

 their internal organs to be seen. 



Recapitulation of Essential 

 Characters. — The body does not 

 exhibit definite segmentation, nor 

 does it carry upon its sides sym- 

 metrically disposed appendages. 

 There is no distinct heart nor 

 blood-vessels, but there is a re- 

 markable set of vessels which 

 usually communicate with the 

 exterior. The nervous system has 

 the form of one or two little ner- 

 vous masses. These characters 

 distinguish the class of the Scole- 

 cida as a whole. 



it the eye, in the form 



Fig. 9.— Diagrnin of the ana- 

 tomy of a Wheel-aiiiraalcule. 

 a Depression in the wheel-or- 

 gan leading to the mouth (6) ; 

 c Dilated upper portion of 

 the gullet, with the horny 

 jaws ; d Stomach ; e Cham- 

 ber into which the intestine 

 opens; gg Water -vessels, 

 opening into contractile 

 chamber ( / ) ; h Nervous 

 system. 



CHAPTER VIII. 



class ANNELIDA. 



The Ringed Worms, or " Annelides," comprise such ani- 

 mals as the Leeches, Earth-worms, Water-worms, Tube- 



