THE CCELOMATA INVERTEBRATA 203 



Finally, the tapeworms, like the sporozoa, exhibit many 

 characteristics which are typical of internal parasites in general. 

 With the abandonment of a free and exposed life they have given 

 up organs of locomotion and protection against foes and evolved a 

 means of maintaining themselves securely in their sheltered position. 

 Their food is supplied in a readily assimilable condition that does not 

 require an alimentary canal or its equivalent, and consqeuently 

 we find that this and its related structures have disappeared. These 

 two points naturally carry with them a corresponding degeneration 

 of the nervous system and absence of sense organs. Although a 

 convenient mode of life, so far as food supply and protection are 

 concerned, internal parasitism implies a difficulty of re-infecting 

 new hosts. The loss of locomotor powers makes the dissemination 

 of the species largely a matter of chance, and as a result a vast 

 number of eggs (or spores in the protozoa) are produced, in order to 

 ensure that some of them at any rate shall reach another host. 

 As a further help in spreading infection we frequently find they 

 adopt the plan of making use of a second or intermediate host, 

 related in some intimate way to the primary host, and this has led 

 in many cases to the development of a complicated life history. 



With Tcenia we close our study of the invert ebrate Coelomates, 

 and pass on to consider the much more highly specialised Chordates. 



