106 Prof. Miiller on the Production 



are necessary to determine the constancy or inconstancy of this 

 attachment, and the mode in which it is effected. And for the 

 present I must leave undetermined how the mollusks make their 

 exit, whether by the spontaneous breaking up of the Synapta or 

 through the spiracula described *oy Quatrefages (which however 

 I have not been able to discover) , or by the continuation of the 

 sac itself to the outer surface." 



It is quite clear then, that whatever the sacs may be, they are 

 not homologous with any normal organ of the Synapta, which 

 clears off one considerable difficulty in the way of the parasitic 

 view. 



The structure of the small third molluskigerous sac mentioned 

 at p. 30 seems worthy of more attention than Prof. Miiller has 

 allotted to it, and may perhaps, if attentively considered, throw 

 some glimmer of light upon the nature of the sacs. 



Prof. Miiller expresses a doubt whether this delicate and short 

 sac was a young undeveloped, or an old retrograding form. 

 However, when we consider " that the ovarian capsule and the 

 contour of the ovary were visible in this sac, but no yelks or yelk- 

 granules" (Archiv, p. 17), or secondary capsules containing yelks ; 

 and further, that in fully developed sacs when the yelks have 

 made their exit the ovarian capsule dehisces (Annals, p. 28) ; 

 it seems pretty certain that it must have been an undeveloped 

 form, and not one which had performed its functions. 



Should this be the case, however, the consequences are highly 

 important. For this small sac contained an " intus-susception " 

 which reached as far as the ovary (Archiv, p. 17), and the intus- 

 suscepted end hung freely in the cavity of the Synapta, while the 

 opposite end was attached to the head of the animal. 



Whether the sac be an organ or a parasite, therefore, it ap- 

 pears that its inner end is at first free, and that eventually it 

 must bore a hole in the intestinal artery and become organi- 

 cally connected therewith ; and this difficulty being equal for all 

 theories may be henceforth eliminated. The fact indeed that the 

 sac is at first attached only to the parietes of the animal and 

 subsequently to its viscera, speaks strongly in favour of its para- 

 sitic nature. We have an exact parallel in the course taken by 

 the " pupse " of Cercaria echinata so ably described by Steenstrup 

 (Alternation of Generations, p. 57 etseq.). The Cercaria buries 

 itself in the skin of the mollusk it infests, loses its active habits, 

 and eventually reaches a particular organ, the liver. Its orga- 

 nization has become simplified in this course; the generative 

 organs apparently becoming developed as the locomotive and 

 digestive organs -retrograde. 



The single case in which sacs were found attached both to the 

 parietes and to the intestinal vessel might be compared to a Di- 



