of Mollmks in Holothurise. 107 



stoma pupa which had just attached itself to the liver without quite 

 leaving the integument, — a stage through which every Distoma 

 must certainly pass, though it has not been actually observed. 



It is only by supposing the adhesion of the molluskigerous sac 

 to the parietes of the Synapta to be a step in its progress, that 

 one can comprehend the two seemingly opposed statements of 

 Prof. Muller, namely that while in at least twenty cases to one 

 the sac is found attached to the intestinal vessel only, in that 

 one instance the adhesion of the sacs to the parietes was so strong 

 that they could only be detached " by repeated and violent tear- 

 ing with needles " (p. 17). 



In the present communication Prof. Muller allows infinitely 

 more weight to the probability of the parasitic nature of the 

 molluskigerous sacs. At page 24 (Archiv), after considering the 

 means of discovering the adult mollusk, we find — 



" Among so many contingences, however, it is to be remem- 

 bered, that possibly our mollusk may never be discovered in the 

 adult state, but that after a short life as such it may cast off 

 shell and operculum and change into a parasitic worm, a herma- 

 phrodite mollusk-generator.'" 



On such an hypothesis it is compared at p. 25 to Lerruea among 

 the Crustaceans ; and taking in addition the two following pass- 

 ages interpolated in the 'Archiv ' at pp. 30-31, we may almost say 

 that Prof. Muller has given in his adhesion to the notion of 

 parasitism. 



The first is inserted after the paragraph containing those very 

 remarkable speculations as to the precedence of hen and egg, &c. 

 " This is nothing more than the logical consequence of con- 

 ceiving the sac to be an extraordinary organ of the Holothuria ; 

 and he, who in the foregoing manner metaphysically explains the 

 observations, only endeavours to define that conception. It need 

 hardly be remarked, that this view is a mere abstract theory 

 (natur-philosophische Doctrin) in the absence of that further 

 knowledge of facts which I desire and seek for." 

 And a little further on we find inserted — 

 " The further investigation of the subject cannot proceed from 

 the conception of its inexplicability, for this excludes all pro- 

 founder knowledge ; we must rather for the present take the very 

 opposite course. Further investigation must proceed upon the 

 basis of what wc know, and explanation must be sought in the 

 common course of nature. 



" According to our present knowledge, a sac which produces 



mollusks can be homologous with nothing but a mollusk, whether 



it arise by an alternation of generations or by a metamorphosis of 



a mollusk. The wonderful connexion of this structure with the 



- Synapta, and always with the same blood-vessel, remains then 



