192 LEISURE-TIME STUDIES. 



theory is neither contrary to facts as we find them, nor is it 

 unsupported by direct evidence. Take, for example, the 

 case of Sacculina (Fig. 20), a well-known parasite, which 

 attaches itself to the bodies of hermit-crabs and their allies. 

 In shape the sacculina resembles a simple sac or bag, 

 a kind of miniature sausage, in fact, which sends into the 

 body of its host a number of root-like processes. These 

 roots entwine themselves amongst the organs of the crab's 

 body, and serve to absorb from the tissues of the host a cer- 

 tain amount of nourishment. If we lay open this curious 

 organism, we find that the sac-like body contains eggs. No 

 traces of structure are discernible ; and but for occasional 

 movements of the body, destined to inhale water and to 

 expel that fluid from its interior, one might regard the saccu- 

 lina as some abnormal growth which had protruded from the 

 body of the crab. The sacculina is a true parasite in every 

 sense of the term. It is dependent, not merely for lodg- 

 ment, but for nourishment also, upon its host ; and, as we 

 shall presently note, its thorough dependence upon the crab 

 becomes the more curious when the past 

 history of the sacculina, as revealed by 

 its development, is duly studied. 



From each egg of the sac-like parasite 

 thus described, a little active creature 

 (Fig. 21) is developed. Known to natur- 

 alists as a "nauplius," the young saccu- 

 lina is seen to be utterly unlike its parent. 

 It possesses an oval body, and is furnished 

 FIG. 21. Young of with three pairs of jointed feet, which 



sacculina. A . 



are used actively as swimming organs. 

 By aid of the long bristles with which the feet are pro- 

 vided, the little sacculina swims merrily through the sea. 

 Its body terminates behind in a kind of forked appendage 

 of movable nature. After the lapse of a short period, 

 changes ensue in the structure of the little body, but there 

 appear as yet no indications of its parasitic origin, or of 

 any tendency to imitate the fixed and attached existence of 



