88 



Dr. J. F. Gemmill. 



[Jan. 17, 



and retractor muscles may be described as acting in converse agreement, in 

 order to produce the swinging part of the movements. That is to say, the 

 anterior protractors and the posterior retractors are associated together alike 

 in relaxation and contraction, while there is a similar harmony in the case of 

 the anterior retractors and the posterior protractors. However, during the 

 powerful downward push of the lantern, which lifts up the urchin, the whole 

 set of protractors must act together. Later, during the backward push of 

 the lantern, the posterior retractors will be in action, while, conversely, the 

 posterior protractors are more or less relaxed. When it is remembered that 

 the protractors tend to close the teeth and the retractors to open them, an 

 explanation will readily suggest itself of the fact that, towards the end of 

 the backward push, the lateral teeth are often seen to be more closely pressed 

 together than the anterior and posterior ones. This point is brought out in 

 fig. 2, and is explained in the notice attached. 



Fig. 2. — Sketch of the teeth and oral membrane towards the very end of the phase of 



protrusion. As in fig. 3, the direction of the urchin's movements was towards 

 the top of the page. The teeth are strongly protruded and point backwards, 

 their tips being brought closely together. The lateral teeth are here shown as 

 coming together more closely than those in front and behind. This condition, 

 like the converse one illustrated in the previous figure, is often seen, and is due 

 to the fact that the posterior retractor muscles alone of the retractor set are 

 now in action, in order to slant the lantern in such a way that the necessary 

 backward push will be given to the tips of the teeth acting against the 

 supporting surface. Meanwhile all of the protractors, except the posterior 

 ones, are in action. It will be remembered that the protractor muscles serve 

 to close the teeth. 



Fig. 3. — Sketch of the teeth and oral membrane as they appear a little after the commence- 



ment of the phase of protrusion. The direction of the urchin's movement was 

 towards the top of the page. At this time the oesophageal papillae (not shown 

 in the sketch) are swollen out so as to fill in great part the space between the 

 teeth. The posterior tooth is here shown as being slightly less divergent than 

 the others. This condition is often seen at the particular stage illustrated. It 

 is due to the fact that, in order to permit the forward swing of the teeth which 

 accompanies retraction, the posterior retractor muscle must have been in a 

 state of at least partial relaxation. It will be remembered that the retractor 

 muscles serve to separate the teeth. 



Fig. 2. 



Fig. 3. 



