372 



capsule, though their fecundation takes place previous to this in- 

 vestment. Upon this succeeds their development. 



The sperm capsules vary from four to eighteen in number, and 

 lie perfectly free in the main sac, not far from the ovary. The 

 spermatic particles, which resemble those of the Gasteropoda, 

 are set free by the bursting of these capsules. 



The development of the egg here proceeds exactly as in the 

 Mollusca, e. g. ActcBon, according to Vogt, and finally assumes a 

 pretty definite character, indicating rather its relation to the 

 Pectini-branchiata. Of its zoological character as a MoUusk, 

 there can, therefore, be no doubt ; and the whole story in a few 

 words, is, that a true Mollusk is developed within a Synapta, not 

 by means of germination, but by the normal sexual products, 

 which occur under otherwise anomalous parts and conditions. 



Such being the facts, the question now arises. What interpre- 

 tation shall be given to these phenomena ? The distinct sexual 

 mode of reproduction would seem to remove them from the 

 catagory of the so-called Alternation of Generations, or Gemmi- 

 parity, as I now understand it. Then, again, the doctrine of 

 " heterogeneous generation," as suggested by Miiller, does not 

 seem to me admissible, beside being particularly unsound; for if 

 an animal can reproduce, by true sexual generation, an ofl^spring 

 zoologically dissimilar to itself. Zoologists may well look about 

 for the stability of their science. 



If I may be allowed an opinion, or rather a view of a subject, 

 on which I have made no observations, I would say that an 

 approximative solution of this enigma seems obtained by admit- 

 ting the possibility of new and hitherto unknown parasitic con- 

 ditions in the life of the Mollusk in question. 



Why may not this Mollusk undergo a form of retrograde 

 metamorphosis, during which its life is parasitic, and very pecu- 

 liarly connected with the life of another and wholly different 

 animal ? Or, again, why may not the phenomena observed, be 

 the final conditions of certain low modes of life, which are con- 

 nected with points in the economy of these animals (Mollusks) 

 that we do not yet understand ? I throw out these remarks in a 

 sufo-estive way. If we refer for a moment to the historical rela- 

 tions of the Cestodes, it will be perceived that there was a time 



