OUR UNBIDDEN GUESTS. 77 



itself to the bodies of hermit crabs. Now, sac-like 

 though this parasite is, and destitute as it is of all 

 the ordinary belongings of animal life, it yet begins- 

 its existence as a little free-swimming animal, exactly 

 resembling a water-flea. The first stages in a saccu- 

 lina' s development are, in short, like the beginnings 

 of the development of some shrimps, of barnacles, of 

 water-fleas, and of crabs themselves, though in a less 

 marked degree. Only after becoming degraded in. 

 structure, does the sacculina become the " guest " of 

 the crab. The mere facts that sacculina is at first as 

 free-living as a fish, and that it afterwards settles 

 down on the crab, testify, if we read nature's story 

 aright, that " once upon a time " the sacculina race 

 was not a parasitic one. Whether or not the sacculina 

 stage itself was the beginning of the attached exist- 

 ence, we do not know. It is most probable that the 

 bag-like body we term a " sacculina " was the result 

 of the adoption of the lower and rooted way of life.. 

 But apart from all other considerations, the main 

 facts that a young sacculina is always free, and that 

 it begins life under a similar guise even to some of 

 the shrimp race, shows that its parasitic life has been 

 acquired, and is by no means an original condition. 



Now the same rule holds good of all " parasites. " 

 The development of most of them shows us the- 

 lingering remnants of a once-free life. But there are 

 other proofs at hand of this assertion. There are 

 degrees and stages in the perfection of the parasitic 

 ' state. There exist animals which are mere " lodgers/' 



