of Mollusks in Holothuriae. 109 



form of Distoma), they attach themselves to their appropriate 

 nidus — a Synapta ; then, undergoing a further development re- 

 sembling in its type partly that of the Anangious Nudibranchiata, 

 partly that of Hectocotylus, they bury themselves in the tegu- 

 ments of the Synapta (a stage answering to the pupa condition 

 of the Cercaria or Distoma) ; that finally, growing and elonga- 

 ting, they attach themselves to the organ for which they are in- 

 tended, and lose more or less their original form (like the Di- 

 stoma) to become little more than mere generative organs ; — sup- 

 posing all this — though it is true that we suppose a great deal — 

 yet there is no mere hypothetical assumption — : not a hair's 

 breadth do we pass beyond the limits of strong and legitimate 

 analogy. 



The discovery of the co-existence of the molluskigerous sac 

 with true generative organs has of course obliged Prof. Miiller 

 to remodel the expression of his views with regard to the bear- 

 ing of his discoveries on the Alternation-theory. Thus we find 

 at page 26 of the ' Archiv ' the following new matter : — 



" The Ho/othurice and mollusks have, beside their calcareous 

 deposits, and the circumstance that certain HolothuriadtE possess 

 a kind of foot from which locomotive suckers can be protruded, 

 no features of resemblance whatsoever. Indeed, according to 

 well-founded ideas, they belong to two different divisions of the 

 animal kingdom. 



" The alternation of generations consists in the succession of 

 two or more generations dissimilar to one another, and of which 

 one is sexual ; — upon a Heterogony, which in some one of the 

 successive generations returns to the earlier form ; — upon a 

 Heterogony, therefore, which is effaced after a regular succession 

 of forms. Thus it is, in the clearly made out ' alternation ' of the 

 Salpce, of many intestinal worms, of the Medusa and Strobila, 

 and of the Aphides. 



" Not quite so certain — at least not absolutely known — is this 

 return in other cases, which seem to be similar, and were in- 

 cluded under the same category by the distinguished originator* 

 of the alternation-doctine — Steenstrup. Of these I shall speak 

 at the end of this essay. 



" Supposing, however, that the phamomena of alternation take 

 place rigidly according to this definition, in all cases, yet it is 

 certain that sometimes two generations occur at the same epoch 

 of an animal's existence, whereof only one produces its like — A 

 from A ; the other produces its unlike — B from A. With these, 

 the phamomena presented by Synapta might be tentatively com- 

 pared. That two sexual generations of different kinds should 



* " Urheber." Prof. Miiller seems to have forgotten his distinguished 

 countryman by adoption — Chamisso. 



