D EG EN ERA TION. 



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to an animal which, like a tapeworm (Fig. 10), obtains its food ready- 

 made in the very kitchen, so to speak, of its host. Hence the lack 

 of a digestive apparatus follows the finding of a free commissariat 

 by the parasite. Organs of sense are not necessary for an attached 

 and rooted animal ; these latter, therefore, go by the board, and the 

 nervous system itself becomes modified and altered. Degradation, 

 wholesale and complete, is the penalty the parasite has to pay for 

 its free board and lodging ; and in this fashion Nature may be said 

 to revenge the host for the pains and troubles wherewith, like the 

 just of old, he may be tormented. Numerous life-histories testify 

 clearly enough to the correctness of the foregoing observations. Take, 

 as an example, the history of Sacculma (Fig. 11, a), which exists as a 

 bag-like growth attached to the bodies of hermit-crabs, and sends 

 root-like processes into the liver of its host. No sign of life exists in a 



Fig. 11. Sacculina and Young. 



sacculina beyond mere pulsation of the sac-like body, into and from 

 which water flows by an aperture. Lay open this sac, and we shall 

 find the animal to be a bag of eggs and nothing more. But trace the 

 development of a single egg, and one may derive therefrom lessons 

 concerning living beings at large, and open out issues which spread 

 and extend far afield from sacculina and its kin. ^ach egg of the 

 sac-like organism develops into a little active creature, possessing three 

 pairs of legs, generally a single eye, but exhibiting no mouth or 

 digestive system parasitism having affected the larva as well as the 

 adult. Sooner or later, this larva known as the naupUus (b) will 

 develop a kind of bivalve shell ; the two hinder pairs of limbs are cast 

 off and replaced by six pairs of short swimming-feet ; while the front 

 pair of limbs develops to form two elongated organs whereby the 

 young sacculina will shortly attach itself to a crab "host." When 

 the latter event happens, the six pairs of swimming-feet are cast off, 

 the body assumes its sac-like appearance, and the sacculina sinks into 

 its adult stage a pure example of degradation by habit, use, and 

 wont. So also with certain near neighbors of these crab-parasites,, 

 such as the Lerneans, which adhere to the gills of fishes. Beginning 

 life as a three-legged "nauplius," the lernean retrogresses and de- 



