IV LUMBRICUS 143 



— into an anterior or head end, carrying the mouth, the sensitive pre- 

 stomium and the cerebral gangha, and a posterior end carrying the anus. 

 Again, the side which is normally uppermost (dorsal side) is difEerentiated 

 from the side which is normally below (ventral). The general symmetry 

 of the worm is bilateral, i.e. with the right and left side alike and equal, 

 in contradistinction to such creatures as Hydroids or Medusae in which 

 there is radial symmetry. Such bilateral symmetry is usual in animals 

 which move actively forwards, while radial symmetry on the other hand 

 is associated with a sessile or drifting habit. Lastly the worm affords 

 a good example of metamerism or metamerie segmentation, i.e. the 

 subdivision of the body into successive somites, each a repetition of the 

 others in its main structural features — body-wall with dorsal pore and 

 chaetae, coelomic compartment, pair of nephridia, nerve ganglion, etc. 



Portions of an animal (or of different animals) whichare morphologically 

 equivalent, built up out of the same elements, are said to be homologous. 

 Thus the fore-limb is homologous in the various types of vertebrate — 

 the fore-leg of a lizard, the wing of a bird, the fore-leg of a dog, the 

 wing of a bat, the flipper of a whale, the arm and hand of a man — 

 these, in spite of their dissimilarity in appearance and in function, are 

 homologous, for they correspond in structure and have arisen in evolution 

 from the fore-limb of the common ancestor. This adjective homologous 

 must be carefully distinguished from analogous which is used to express 

 functional not structural correspondence. Thus the wings of a Fly and 

 a Bird are analogous organs for they serve the same function but 

 they are not homologous for there is no structural or evolutionary 

 correspondence. 



Again, a nephridium of one worm is homologous with that of another. 

 Even within the body of the same animal organs may be homologous, 

 e.g. organs on one side of the body are homologous with their fellows 

 on the other. Or in the case of a metamerically segmented creature 

 they may be homologous with their representatives further forward or 

 back in the series. In this case the expression serial homology (or 

 homodynamy) is used : thus the individual somites of the worm are 

 said to be serially homologous. 



The last great difference between the worm and the coelenterate is 

 that whereas the latter has, interposed between the two primary layers 

 of cells, a mere structureless mesogloea, with at the most a few scattered 

 immigrant cells, the worm has on the other hand interposed between 

 ectoderm and endoderm the complicated arrangements of tissue consti- 

 tuting the mesoderm and mesenchyme. From the worm upwards, 

 throughout the animal kingdom, these constitute, as indicated at the 



