386 EVENINGS AT THE MICROSCOPE 



by gradual and almost imperceptible diminution of the 

 circumference. 



Around this expansile, but now fast-closed mouth, you 

 observe four tentacles, radiating, in a plane at right angles 

 to the axis of the thread, toward the four cardinal points; 

 they are long, slender, straight, and each is terminated by 

 a globose head of considerable size, resembling the arms 

 of certain screw-presses, which are loaded with terminal 

 globes of metal to increase their impetus when turned. 



The structure of these tentacles is very interesting. 

 The stem contains a core or central chain of large cells, 

 which take a somewhat square outline from mutual press- 

 ure. The surface is roughened with small swellings, from 

 each of which projects a long and excessively attenuated 

 hair (palpocil) , which is probably a very delicate organ of 

 touch. The terminal globe is filled with proportionally 

 large oval vesicles, each with a central cavity, which are 

 arranged in a divergent manner around the centre, so that 

 their tips shall reach the surface of the globe; these are 

 those potent weapons of offence called thread-cells (cnidce). 

 The surface of the globe is covered with short thick pal- 

 pocils, which Dr. T. S. "Wright considers as prehensile 

 organs. ''These palpocils arise, each as a somewhat rigid 

 process, from the side of one of the large thread-cells 

 buried in the head of the tentacle; and they probably 

 convey an impression, from bodies coming into contact 

 with them, to the thread-cell, causing the extrusion of 

 its duct." 



Besides these globe-headed tentacles, there are, on the 

 lower part of the club-foot, four other organs similar in 

 every respect, except that they are not furnished with 

 heads, nor any terminal dilatation whatever. They pro- 



