CHAPTER VII 

 THE CCELOMATA INVERTEBRATA 



The Earthworm, Lumbricus sp., a Free-living Annelid Tcenia solium, 

 a parasitic flat worm. 



THE next grade of animal organisation above the Ccelenterata is 

 the Coelomata, and it includes all the remaining animals with a few 

 possible exceptions. The main point of difference between them is 

 that in the latter not only do we find a gut cavity or enteron lined 

 by entoderm, but in addition, between it and the outside layer, the 

 ectoderm, is another cavity, the coelom, whose walls are lined by a 

 third cellular layer, namely, the mesoderm or middle layer. Thus 

 there are two independent cavities, enteron and coelom, and the main 

 structural plan of such an animal reduced to its simplest form is that 

 of a narrow tube, the gut, within a wider one, the body wall, both 

 being joined together at the two ends. In addition to this funda- 

 mental difference there are a number of smaller ones which can be 

 better dealt with after we have examined the structure of a primitive 

 coelomate. 



The Earthworm Lumbricus sp., a Free-living Annelid. 



Earthworms are widely distributed over the face of the earth, 

 and are found in almost all places where there is a certain amount of 

 moisture. Of the twenty or more species common in the damp soil 

 of this country the two largest and commonest belong to the Genera 

 Lumbricus and Allolobophora. They differ from one another only in 

 unimportant points, and for convenience in the laboratory we use 

 the largest of all the British species, namely, L. herculeus. They 

 burrow into the ground by literally swallowing the soil in front of 

 them, from which, as it passes through their alimentary canal, they 

 obtain their food, consisting of decaying animal and vegetable 

 matter. It is this earth which is constantly being passed through 

 their bodies, even when upon the surface, that produces the character- 

 istic " worm casts " which spoil the appearance of a lawn or the 

 surface of a putting green. The worm plays a far more important 

 part in the biology of the soil than appears at first sight, and is 



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