HOW DO STARFISHES OPEN OYSTERS ? 269 



Astropeden and Astcrias possess tube-feet of very different structure. 

 Those of Astropeden are conical and quite pointed at the end, and seem 

 extremely well adapted to boring in sand. Suckers at their ends are 

 entirely wanting. Such enlargements would only be a hindrance when 

 boring in sand. On the other hand, Astropeden has no need of 

 suckers, for it does not climb steep walls ; the animals which it preys 

 upon all move so slowly that they could not escape by flight, and there- 

 fore do not require to be held fast ; and thirdly, this starfish does not 

 need to open its victims. With its feet it brings them into the capacious 

 stomach, from which they cannot again escape. It has now only to 

 quietly wait its time, until the animals, killed by suffocation, open their 

 shells and allow the digestive juices to reach them. 



In the case of Asierias the circumstances are quite different. The 

 animal is a zealous climber, and by preference clings to perpendicular 

 walls. If, like Astrop)ecten, it possessed pointed tube-feet without 

 suckers, it could not do this, but would fall down as Astropecten does 

 when it attempts to climb in confinement. The animals which Asterias 

 eats are some of them capable of relatively rapid locomotion, and 

 therefore require to be held fast. Many of them, too, have the power 

 of tightly closing their shells, and if the Asterias wishes to get at their 

 soft bodies, the shell must first be opened. For clinging, as well as for 

 opening shells, pointed feet would be quite useless. Feet, however, 

 provided with powerful suckers, such as Astericis possesses, are well 

 adapted to these uses. 



There is, too, a difference between the ways in which the feet of 

 Astropecten and Asterias move. Whilst Asterias, when the suckers 

 have been loosened, curves the feet outwards, and so draws itself back, 

 Astropecten curves them inwards. It is obvious that the latter mode 

 of progression is much better adapted to a life buried in sand, such 

 as Astropecten leads. 



Different views have been expressed as to the manner in which 

 Asterias and similar forms succeed in opening the shells of molluscs. 

 At the present time it seems to be generally considered that this is 

 accomplished by the secretion of a stupefying fluid, or poison. As 

 we shall see, however, further on, this view is a complete mistake. 



In what follows we shall consider (1) the possible methods by which 

 the opening of the shell could be accomplished ; (2) which of these is 

 to be considered the most probable ; and, finally (3), we shall endeavour 

 to prove that this method is, in fact, adopted. There appear to be 

 altogether six possible ways : — 



1. The starfish might take the molluscs hy surprise. — Bivalves, includ- 

 ing the oyster, are generally very watchful. A small change of light, 

 a shadow, a slight movement of the water, or any trifling disturbance, 



