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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



fishes which live within the bodies of large sea- 

 anemones, and of other organisms, and which 

 swim in and out at will, obtaining their food for 

 the most part from the external world. A sim- 

 ple modification of habit in such animals would 

 convert them into true parasites. Suppose that 

 the guest finds that it may readily obtain food by 

 living on the matters its host is elaborating for 

 its own use, and suppose, further, that the ani- 

 mal-guest gradually accommodates itself by suc- 

 cessive modifications to its new mode of life, and 

 we have thus the influence of habit brought into 

 play and exercised upon the descendants of the 

 first parasite in producing a literal race of such 

 beings. Such a belief or theory is neither con- 

 trary to facts as we find them, nor is it unsup- 

 ported by direct evidence. Take, for example, 

 the case of Sacculina, 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 en- 

 twine themselves among the organs of the crab's 

 body, and serve to absorb from the tissues of 

 the host a certain 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 sacculina 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 lodgment, 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 is developed. 

 Known to naturalists as a " nauplius," the young 

 sacculina is seen to be utterly unlike its parent. 

 It possesses an oval body, and is furnished with 

 three pairs of jointed feet, which are used active- 

 ly as swimming-organs. By aid of the long bris- 

 tles with which the feet are provided, the little 

 sacculina swims merrily through the sea. Its 

 body terminates behind in a kind of forked ap- 

 pendage 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 indica- 

 tions of its parasitic origin, or of any tendency 



to imitate the fixed and attached existence of its 

 parent. The body of the young sacculina next 

 becomes folded upon itself, so as to inclose 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 fila- 

 ments 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 sac- 

 culina. 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 them- 

 selves 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 con- 

 verted 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 regarding its assumption of 

 a parasitic life. The most reasonable view which 

 can be taken of the development of an animal or 

 plant is that of regarding the phases of its pro- 

 duction as presenting us with a condensed or pan- 

 oramic picture of the stages through which it has 

 passed in the course of its origin or evolution 

 from some preexisting form. If we refuse to re- 

 gard development in this light, the stages through 

 which the living being passes in its progress 

 toward maturity present themselves as a set of 

 unmeaning and wholly inexplicable actions and 

 conditions. While, on the other hand, when we 

 recognize that in the development of an animal 

 we may trace its ancestry, much that is otherwise 

 incomprehensible becomes plain and reasonable, 

 and very discordant phases of life become harmo- 

 niously adjusted through such a consideration. 

 And when we further discover that a large num- 

 ber of animals, widely differing from each other 

 in their adult structure, exactly resemble each 

 other in their young state, the feasible nature of 

 the statement, that such a likeness implies a com- 

 mon origin, is readily demonstrated. On any 

 other supposition, in short, the development of 

 living beings presents us with phases of utterly 

 unintelligible nature. Now the young sacculina 

 is found to present a close resemblance to a large 

 number of other animals belonging to the great 

 class known as the Crustacea. To this group be- 



