34 Prof. Miiller on the Development 



for all these are spasmodically contracted at their extremities, so 

 that nothing can either pass from or into that cavity with its 

 normally-contained saline fluid. Besides, how could a thousand 

 or more mollusks creep in, particularly as they must have entered 

 as yelks ? 



Neither have they crept into the sac from without, since they 

 have arisen from their elements in it. It follows then that the 

 sac must either itself be the equivalent of a mollusk, a vermiform 

 metamorphosis of a mollusk as it were, which has made its way 

 into the Holothuria ; or it must be an organ of the Holothuria, 

 which instead of Holothurice produces mollusks. If the mol- 

 luskigerous sac be itself an animal, the intus-susception must be 

 regarded as an intestine, the interior of the sac as an abdominal 

 cavity, the ovarium and the sperm-sacs as the generative organs 

 of the animal. The whole difficulty, however, does not consist 

 in conceiving the sac to be an animal. 



A grand difficulty for every theory is, that the molluskigerous 

 sac is organically connected with the Holothuria. The knob-like 

 end is not merely adherent by a sucker, or otherwise, to the Ho- 

 lothuria and its vessel, but the vessel of the Holothuria embraces 

 and is grown to the knob of the sac. 



Has this sac, then, perhaps arisen as a bud in the Holothuria, 

 remaining in connexion with it, and perhaps having the same 

 relation to the production of the mollusks as the proembryo of 

 certain plants has to their production ? Against this view, how- 

 ever, we have the fact, that the sac opens at the same place as 

 the ordinary generative organs of the Holothuria. 



Perhaps it is a case of the alternation of generations, the Ho- 

 lothuria producing mollusks, from which again Holothurice are 

 produced, though it is highly improbable that the alternation of 

 generations ever goes so far ; and besides, the Holothuria has its 

 own peculiar mode of reproduction, its own ova, with whose pro- 

 duct indeed we are not yet acquainted, but which indubitably is 

 wholly different from a mollusk, and without question is again a 

 Synapta. 



The mollusks are produced only in certain rare individuals of 

 the Holothuria, which, instead of the normal generative organs, 

 have others specially adapted for the production of mollusks. 



Again, the essence of the alternation of generations is, that the 

 form B, produced from, and dissimilar to, A, reproduces the form 

 A. How would it be, however, if B propagated itself as B, and A 

 as B, but also as A ? Such a possibility had long since presented 

 itself to my mind ; and it seemed to me, that in the further 

 development of the phsenomena of alternation lay the possi- 

 bility of an insight into the mode in which new forms have been 

 introduced into the creation. From this point of view, then, the 



