THE SPHAEROBOLUS GUN 341 



the cavity formed between the peridial membranes opens to the 

 outer air at the rim of the cup between the points of the teeth, i.e. 

 at each tooth-sinus (Fig. 161, A). This being so, it is obvious that, 

 before the discharge of the gun, an arrangement is provided by 

 which it is possible for air to pass from the outer atmosphere into 

 the peridial cavity. As discharge of the gun takes place and the 

 inner peridial membrane becomes everted, the tooth-sinus apertures 

 increase in size and finally become diamond-shaped, as shown in 

 Fig. 147, C (p. 298). 



We have seen that when a Sphaerobolus projectile is shot 

 vertically upwards to a maximum height of 7 feet the eversion of 

 the inner peridium takes place in about one-thousandth of one second, 

 and that when it is shot vertically upwards to a maximum height of 

 14-5 feet the eversion of the inner peridium takes place in about 

 one-fifteen-hundredth of one second. This exceedingly rapid eversion 

 of the inner peridial membrane necessitates a correspondingly 

 rapid enlargement of the cavity between the inner and outer peridial 

 membranes ; and this exceedingly rapid enlargement of the peridial 

 cavity must tend to cause the formation of a partial vacuum within 

 the cavity's interior. Now, if the inner and outer peridial mem- 

 branes were completely attached to one another all around the rim 

 of the stellately opened cup instead of only at the points of the 

 teeth, as discharge of the gun took place the inner peridial membrane 

 during its eversion would have to do its work against the pressure 

 of the atmosphere ; but, owing to the existence of the tooth-sinus 

 apertures, which are so constructed that they enlarge as discharge 

 takes place, this mechanical difficulty does not arise. It is obvious 

 that, when the discharge of a Sphaerobolus gun takes place, a blast 

 of air must rush into the enlarging peridial cavity through every 

 tooth-sinus aperture, so that any slight negative air-pressure which 

 may be formed in the cavity is quickly reduced to zero. From this 

 discussion we may conclude that the tooth-sinus openings between 

 the inner and outer peridial membranes have the function of per- 

 mitting of the rapid entrance of air into the central peridial cavity 

 during the discharge of the gun. 



The enlargement of the peridial cavity during the discharge of 

 the Sphaerobolus gun may be roughly estimated at 4-6 cubic mm. 



