and its supposed Parasite, 219 



cumstance to be considered, which has been referred to this 

 very point. 



In one case described by J. Miiller, which, among the rare 

 cases of cephalic attachment of the molluskigerous sac, was itself 

 a rare exceptional case, inasmuch as three sacs were present in 

 one Synapta, and all three of them were attached to the head, 

 one of the sacs, and this the smallest and shortest, although 

 otherwise not different, and still uninjured, had the extremity 

 usually affixed to the intestinal vessel free in the cavity of the 

 body. J. Miiller was inclined to attach much importance to 

 this circumstance in the interpretation of the sacs as parasitic 

 creatures. The mode in which this might happen has been 

 elucidated by the anonymous reporter in the ' Annals of Natural 

 History,' January and February 1852, by reference to the pene- 

 tration of the Cercarise into the skins of the Mollusca. J. Miiller 

 himself has left it uncertain whether this third sac, with a reversed 

 attachment (that is to say, attached by the usually free ex- 

 tremity, and free at that which is usually attached), was still very 

 young and undeveloped, or whether its development and genera- 

 tion were completed, and itself in course of retrograde metamor- 

 phosis, — whether it would attain its position on the intestine 

 only by further growth, or whether the union had previously 

 occurred at the ordinary place, but been dissolved by the gradual 

 reduction of the sac. The reporter in the * Annals of Natural 

 History' regarded it as an ascertained fact, from J. Miiller's ob- 

 servation, that the sac is at first attached only to the body- wall 

 of the Synapta (at the head), and subsequently to the intestinal 

 vessel. 



The behaviour of the molluskigerous sacs as they occur in the 

 cavity of the body of the Synapta, especially when the cephalic 

 attachment is left out of the question, forbids our connect- 

 ing this circumstance with evolution, whether progressive or 

 retrograde, or with the immigration of the molluskigerous sac 

 It is quite an ordinary phenomenon to find in the cavity of the 

 Synapta perfectly free, floating molluskigerous sacs, which have 

 evidently been torn from the intestinal vessel only by the cap- 

 ture and the violent movements of the Synapta accompanying 

 it. But if a molluskigerous sac be completely attached to the 

 head, the freedom of the other extremity is still less remarkable. 

 As it may be proved from the cephalic attachment itself that in 

 this case a force has been exerted upon it which pressed it 

 against the head of the Synapta, and even wedged it into that 

 part, it is not to be wondered at if the same pressure which, in 

 the case of J. Miiller, drove these sacs towai'ds the head, should 

 have separated one of them from its position on the intestinal 

 vessel. Nor is it surprising that this should apply to the 



