218 M. A. Baur on Synapta digitata 



extends backward in the body-cavity of the Synapta, in this 

 case it turns forward towards the head, its extremity is forced, at 

 any spot offering the least resistance, between the parts there 

 existing, and under certain circumstances into the canals, where 

 it is firmly held, like a portion of intestine wedged into a diver- 

 ticulum of the ventral cavity : such a spot is presented by the 

 tentacular cavities, if we leave out of consideration the delicate 

 and but slightly resistant walls of the aquiferous vessels, by 

 which the tentacular cavities are separated from the body-cavity 

 of the Synapta. 



When disquieted or irritated, the Synapta, or each fragment 

 having a head, endeavours to break itself to pieces. Before this 

 is effected at any definite spot, the worm-like body performs 

 powerful alternate contractions and extensions. The pressure 

 exerted by the muscular body-wall upon its contents is so strong 

 that, at the moment when the wall bursts, the fluid of the body- 

 cavity often spirts out in a stream. At the same time it 

 frequently happens that with the first rupture of the body-wall 

 the intestine, together with the genital tubes, is forcibly driven 

 out laterally. In the same way the molluskigerous sac, with its 

 free end turned towards the head, may be firmly held by a con- 

 striction in the neighbourhood of the calcareous ring, where a 

 rupture never takes place. The attenuation which, according to 

 J. Miiller, the sac exhibits towards its extremity is explained 

 by the dragging and stretching necessarily accompanying this 

 process. 



As appears from the nature of the Synapta, as well as from 

 that of the extremity of the sac, the mode of attachment is purely 

 mechanical ; it does not consist in an organic connexion of the 

 sac with the head of the Synapta. In the effectuation of the 

 cephalic attachment the molluskigerous sac is purely passive : 

 the effective cause is the pressure exerted by the contractile wall 

 of the body of the Synapta upon its contents, and consequently 

 also upon the molluskigerous sac. 



The cephalic adhesion is, finally, inconstant, and purely acci- 

 dental, because it is to be ascribed to an accidental vital mani- 

 festation, or perhaps more correctly to a phenomenon of the 

 agony of the Synapta— a. convulsive contraction of the body, 

 perhaps produced only by capture or by violent injury. 



The notion that we are to regard the molluskigerous sac, 

 when attached to the head of the Synapta, as not organically 

 united, but perhaps as a parasite breaking through the body-wall 

 of the Synapta either in immigration or emigration, and the 

 adhesion itself as a transitory act of the molluskigerous sac 

 which, for this reason, so rarely comes under observation, is got 

 rid of by what has been said above. But still there is one cir- 



