224 ENTO-PARASITES AND HYGIENE. 



parasites the alimentary canal may be dispensed with 

 entirely, and that all its functions are assumed by the 

 outer skin. The great number of cestods do not possess 

 the slightest trace of it, their outer surface being stomach 

 and intestine at the same time, a condition readily con- 

 ceiv^able wljen we keep in view that they live in a place 

 filled with nutritious material that can be absorbed 

 at once without the least amount of energy on the part of 

 the i^arasites. 



So we discover in all these animals a gradual simplifi- 

 cation of their outer and inner bodily structure, which 

 must be regarded as a direct and immediate consequence 

 of their mode of living. This explains fully why all 

 parasites, whatever their original shape and form may 

 have been, possess such a simple and similar appearance. 



We must, however, not lose sight of the fact that 

 there are also influences of a progressive nature, and it 

 will be necessary to examine them and their relations to 

 the life of the individual also. They are prominently 

 connected with the preservation of the animals, and con- 

 sist mainly of appliances for getting a firm hold, for 

 clinging to the place where they have become sessile. 

 Temporary parasites did not need them, for they were 

 provided with organs of sense and of locomotion, so that 

 in case they were removed from their host it was a sim- 

 ple thing for them to get back. Yet even among these 

 we may find a remarkable development of hooks and 

 clamps, because the individual wants to guard itself 

 against an unnecessary removal from the seat of life {e. 

 g.^ leech, etc.) Such an occurrence would, of course, be 

 a very serious mishap to the more sessile parasites which 

 have no organs of locomotion ; consequently, in exact 

 proportion to the reduction of their locomotory appara- 

 tus, their organs of clinging are developed to a much 

 greater state of efficiency. In the beginning, those 

 organs already on hand are only strengthened ; hooks 



162 



