220 M. A. Baur on Synapta digitata 



smallest specimen — to that whose length is less than the distance 

 of the ordinary point of attachment on the intestine from the head 

 of the Synapta, as may be seen from the figure (tab. 2. fig. 3) given 

 by J. Miiller. Therefore, as the abnormal attachment to the 

 head is nothing but a mechanical and accidental engagement of 

 one end of the sac, so the simultaneous absence of the normal 

 adhesion in the one case observed by J. Miiller is merely a me- 

 chanical separation of the other end from the intestinal vessel, 

 referable to the same accidental cause. Neither the one nor the 

 other stands in any nearer relation to the growth, development, 

 or vital history of the molluskigerous sac. 



II. On the adhesion of the Molluskigerous Sac to the Intestine. 



In the natural state we are never to imagine the molluskigerous 

 sac in the Synapta otherwise than as attached by one end, in 

 the manner described and figured by J. Miiller, to the intestinal 

 vessel opposite to the mesentery, with the other end extending 

 freely backwards in the cavity of the body along the intestinal 

 canal of the Synapta. If the sac be quite free, it has been acci- 

 dentally separated ; if it be attached by the free end to the head, 

 it has been accidentally engaged there. 



The natural adhesion of the molluskigerous sac is effected in 

 this way : the button-like extremity, probably the buccal portion 

 of the molluskigerous sac, projects into the lumen of the intes- 

 tinal vessel through a slit in the wall of the latter, and is firmly 

 held therein, exactly like a button in its button-hole. Behind 

 the knob the wall of the vessel clasps the sac so closely, that a 

 forcible separation is not possible without tearing away the 

 button, or enlarging the slit, so that the sac may easily appear 

 to have become organically united with the intestinal vessel. 

 There is, however, no evidence of such a union ; on the con- 

 trary, the simple engagement of the button is perfectly sufficient 

 to explain the firmness of the connexion between the sac and 

 the intestinal vessel. 



The mode of attachment of the molluskigerous sac to the in- 

 testinal vessel is, therefore, like its adhesion to the head, purely 

 mechanical, and depends no more than the latter upon an orga- 

 nic union. It is not, however, like this, accidental ; on the con- 

 trary, this attachment is constant for the occurrence of every un- 

 injured sac in the uninjured Synapta, and, which is the most 

 remarkable, constant with regard to the spot on the intestinal 

 vessel, although this extends, with the same diameter, and the 

 same properties, and in the same position, along the whole intes- 

 tine of the Synapta, from the stomach to the anus. 



How this attachment is effected, that is to say, how the mol- 

 luskigerous sac buttons itself into the intestinal vessel of the 



