3 68 



THE POPULAR SCIENCE MONTHLY. 



C A 



has no peculiarities to indicate that the structure and habits of the 

 adult are to be in any way strange or unusual ; but after a time it finds 

 its way, in some manner which has not been clearly made out, into the 

 body of an holothurian, and this change in its habits is accompanied 

 by a most marvelous change in its organization. It now has no shell, 

 since the body-wall of its host affords ample protection, and, as it no 

 longer needs to change its position in search of food, or to escape its 

 enemies, its foot and specialized muscles have disappeared. Its organs 

 of sense are wanting, and the nervous system is so rudimentary that 

 no traces of it can be discovered. It has no need of organs to capt- 

 ure, masticate, or digest its food, since it sucks this, already digested, 

 from the stomach of its host, and its digestive system has accordingly 

 become reduced to a simple pouch, with only one opening the mouth 

 and, as the whole surface of the body is bathed by the blood which 

 is aerated in the respiratory organs of the holothurian, it has no need 

 for gills, or heart, or blood-vessels, and, so far as our knowledge goes, 

 these organs are entirely lacking. 



Of the highly specialized organs of a gas- 

 P teropod only the simple stomach, the reproduc- 

 / tive organs, and the slightly muscular body- 

 wall remain, and no person who is not ac- 

 quainted with the fact that young snails with 

 spiral shells have been seen to come from its 

 eggs would suspect that entoconcha is a mol- 

 lusk. 



Such cases, which modern research has 

 proved to be by no means infrequent, show 

 that the comparative study of adult animals 

 can not furnish a complete key to their past 

 history, and they also illustrate how little is to 

 be hoped for from paleontology. 



No one who accepts the doctrine of descent 

 with modification, and is familiar with the em- 

 bryology of the gasteropods, can doubt that, if 

 we were able to trace back the pedigree of ento- 

 concha, we should be led to a remote ancestor 

 which was an ordinary gasteropod ; not neces- 

 -~ n sarily a species exactly like any we know, but 

 a form with general gasteropod characteristics 

 at least : nor can we doubt that, if we were able 

 to study the embryology of this ancestral form, 

 which we may represent by the letter A in diagram 3, we should find its 

 life, from the egg to maturity, to be made up of a series of stages, a, b, 

 c, d, e,f, etc., substantially like stages in the life of ordinary gasteropods, 

 B and C. The relation between the entoconcha of the present day, D, 

 and this gasteropod ancestor, is shown by the curved line to D. Start- 



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