PARASITES AND THEIR DEVELOPMENT. 193 



its parent. The body of the young sacculina next becomes 

 folded upon itself, so as to enclose the young animal in a 

 more or less complete manner ; and the two front limbs 

 become developed beyond the other pairs, and form large 

 organs wherewith the little creature may ultimately moor 

 itself to some fixed object. From the extremities of these 

 altered fore limbs two elongated processes or filaments are 

 seen to sprout, and these processes are regarded as the 

 beginnings of the root-like organs seen in the attached, 

 parasitic, and full-grown sacculina. The other two pairs of 

 feet are cast off, and in their place six pairs of short 

 swimming feet of forked shape are developed. After this 

 stage has been attained, the young animal seeks a crab-host ; 

 the root-like front feet attach themselves to the body of the 

 crab and penetrate into its substance; the other feet are 

 cast away as useless organs; and with the assumption of 

 the sac-like body, the young sacculina becomes converted 

 into the likeness of the parent-form. 



Such is a brief sketch of the development of a true 

 parasite, and we may now inquire what the life-history of 

 this animal teaches us concerning its antecedents, and re- 

 garding its assumption of a parasitic life. The most reason- 

 able view which can be taken of the development of an 

 animal or plant is that of regarding the phases of its produc- 

 tion as presenting us with a condensed or panoramic picture 

 of the stages through which it has passed in the course of 

 its origin or evolution from some pre-existing form. If we 

 refuse to regard development in this light, the stages through 

 which the living being passes in its progress towards maturity 

 present themselves as a set of unmeaning and wholly inex- 

 plicable actions and conditions. Whilst, on the other hand, 

 when we recognise that in the development of an animal 

 we may trace its ancestry, much that is otherwise incompre- 

 hensible becomes plain and reasonable, and very discordant 

 phases of life become harmoniously adjusted through such a 

 consideration. And when we further discover that a large 

 number of animals, widely differing from each other in their 



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