SOCIAL AMD COMMUNAL LIFE 4*1 



Because they have no need of locomotion they have no 

 need of organs of orientation, those special sense organs 

 like the eyes, ears, and feelers which serve to guide and 

 direct the moving animal ; and most fixed parasites will 

 be found to have no eyes, or any of those organs acces- 

 sory to locomotion, and which serve for the detection of 

 food or of enemies. Because these important organs, 

 which depend for their successful activity on a well-organ- 

 ized nervous system, are lacking, the nervous system of 

 parasites is usually very simple. Again, because the 

 parasite usually feeds on the already digested food or the 

 blood of its host, most parasites have a very simple ali- 

 mentary canal, or even none at all. Finally, as the fixed 

 parasite leads a wholly sedentary and inactive life, the 

 breaking down and rebuilding of tissue in its body goes 

 on very slowly and in minimum degree, so that there is 

 little need of highly developed respiratory and circulatory 

 systems; and most fixed and internal parasites have these 

 systems of organs decidedly simplified. Altogether the 

 body of a fixed permanent parasite is so simplified and so 

 wanting in all those special structures which characterize 

 the active, complex animals that it often presents a very 

 different appearance from those forms with which we know 

 it to be nearly related. This simplicity due to loss or 

 reduction of parts is called degeneration. Such simplicity 

 of body-structure due to degeneration is, however, essen- 

 tially different in its origin from the simplicity of the lower 

 simpler animals. In them the simplicity of body is prim- 

 itive; they are generalized animals; the simplicity of 

 degeneration is acquired; it is really an adaptation, or 

 specialization. 



An excellent example of body degeneration due to the 

 adoption of a parasitic habit is that of Sacculina (fig. 159), 

 a crustacean parasitic on other crustaceans, namely, crabs. 

 The young Sacculina is an active, free-swimming larva 



