234 Elementary Biology. 



the year, the serpentine motion, and perhaps even the exist- 

 ence of the rows of bristles on its under surface by which 

 that movement is facilitated are likewise common know- 

 ledge. 



It will be remembered that in Chap. III. sect. i. we dis- 

 cussed the differentiation of organs according to the duties 

 which they severally had' to perform ; and we saw that it 

 was possible to classify these organs according to the share 

 they had in collecting, assimilating, and distributing the food 

 (p. 51). It will be found most convenient if we adopt this 

 classification in the discussion of the morphology of the 

 animals with which we have yet to deal. In the types of 

 plant life which we considered, and in the Hydrozoa, the 

 examination of which we have just concluded, it was not 

 possible nor advisable to treat of the various organs sepa- 

 rately from each other according to that plan ; for what 

 particularly strikes us in these types is the want of that 

 extreme differentiation of organs which we shall afterwards 

 see is so characteristic of the higher animals. The leaves 

 were the organs of respiration as well as of nutrition, in both of 

 which functions the stem and branches, if green, participated. 

 The branches, stem, and roots were the organs of circulation, 

 but they were also organs of support, and frequently played 

 an important part in nutrition, in addition to that already 

 referred to. Indeed it might be said that the highest plant 

 is far below an animal even so low as the worm, because of 

 this want of differentiation, although the power the worm 

 possesses of replacing lost parts also points to a lowness of 

 organisation, expressed in the plant by the power it likewise 

 possesses of reproducing itself by buds, shoots, and cuttings, 

 and of giving origin to new leaves and branches when the 

 former ones are removed. We have just seen that among 

 the lowly organised Hydrozoa the same characters are 

 prominent. 



We may best commence our examination of Lumbricus 

 by a survey of its external characters. The animal varies 



