and its supposed Parasite. 217 



kigerous sacs were smaller, but they were rather amongst the 

 largest observed ; they were all sexually mature, and contained 

 either ready-formed mollusks or ova in course of development 

 into mollusks. None of the three cases furnished any support 

 for the supposed possibility that the sacs attached to the head 

 of the Synapta might be an earlier stage. 



The connexion of the extremity of the sac with the head of 

 the Synapta, when it exists, is very intimate. The terminal 

 portion of the molluskigerous sac, when forcibly pulled, is torn 

 before the adhesion will yield. This firmness is not due to any 

 coalescence ; its cause is mechanical, and depends upon an inter- 

 locking. The terminal portion of the molluskigerous tube was 

 in all three cases wrapped together into the form of a coil — as it 

 were, stuffed into a space enlarged by stretching, and held firmly 

 therein by a narrow constricting portion. After the careful 

 separation of the constricting parts in the head of the Synapta, 

 the end of the sac could be completely unfolded and prepared 

 in an uninjured state, with the exception of the loss by stripping 

 off of the outermost layer of cells. The point of attachment 

 always corresponds externally to a spot of the circumference of 

 the oral or cephalic disk, where it borders on the base of the 

 tentacles. The point of attachment is not constant upon this 

 circumference; it is sometimes on one side and sometimes on 

 the other of the median line indicated by the mesentery and 

 sexual orifice. In two of the observed cases a yellow swelling, 

 caused by the more intensely coloured molluskigerous sac shining 

 through it, could be detected even from the outside in the still 

 uninjured and living head of the Synapta within the circle of 

 tentacles. The body-wall of the Synapta was, however, not 

 perforated ; the end of the sac was not bare, but was always 

 distinctly covered, at least, by the outermost reddish layer of the 

 body-wall (epidermis and dermis of Quatrefages) . In one case 

 the most careful examination showed that internally the coil- 

 like end of the molluskigerous sac had filled and considerably 

 distended the base of two neighbouring tentacular cavities. 

 These remained distinctly enlarged even after the separation 

 and removal of the coil. In the two other cases the end of the 

 sac had not occupied the cavity of the water-vascular system at 

 its entrance into the tentacles, but the space close by, between 

 the buccal disk, calcareous ring, and oesophagus, and therefore 

 between the canals passing from the annular canal to the 

 tentacles. 



The condition described, especially in the first case, in which 

 the entrance into two tentacles was evidently forcibly dilated, 

 admits only of one explanation. The adhesion of the sac to the 

 head can be effected only in the following way : — whilst it usually 



