THE JELLYFISH AND THEIR ALLIES 3455 



of the skin, known as the cnidocil. Within the vesicle is found, either spirally 

 rolled or in an irregular tangle, a long filament, or hollow tube, which is a prolonga- 

 tion of the vesicle, but turned outside in. This tube, which is more than twenty 

 times as long as the cell, is pointed at the tip, and almost up to the tip beset with 

 two rows of fine, spirally-arranged, barbed hooks. When the cnidocil is touched 

 or irritated, this filament is violently shot forth, being turned inside out like the 

 finger of a glove. So long as the thread remains rolled up within the vesicle, 

 the barbed hooks are, of course, in the tube, but when it is shot out, they come 

 on the outside. The rolled-up thread appears to be filled with some poisonous 

 substance, which, when the tube is shot out, is ejected over the spot where 

 the point strikes and wounds. It has been asserted that in many, probably in 

 all, Coelenterates, muscle and nerve elements occur below, and are associated with, 

 the stinging cells. The action of the stinging cells is perhaps as follows: The 

 cnidocil is touched by some passing object, and conveys the stimulus which 

 leads to the violent contraction of the distended vesicle. This forces out the tube 

 which is lying in it, in the manner described. A simple touch is, however, not 

 enough to cause the cell to be discharged, otherwise such discharge would take 

 place when the animal knocks against a stone, or when the tentacles, being with- 

 drawn, touch the body. We must assume that the Cnidaria are able to distinguish 

 between the various stimuli received from contact with other bodies. The stinging 

 cells are very often grouped together to form so-called stinging batteries of various 

 sizes. Dr. Mobius writes that as soon as the capturing arm touches the passing 

 victim, the long filaments are shot out of the stinging capsules, penetrate or adhere 

 to the animal and detain it. Unless the prey is stronger than its attacker, it cannot 

 escape. New filaments are being continually shot out at it as it is slowly drawn in 

 toward the mouth; even within the body cavity similar stinging cells are found in 

 the skin. The greater the struggle, the larger the number of capsules discharged, 

 in order to hold the prey. 



THE JELLYFISH AND THEIR ALLIES Class Polypomedusse 

 Order SIPHONOPHORA 



We have already described the swimming bells of the jellyfish as the highest 

 development of the stinging group. The Siphonophora, as represented by the 

 Portuguese man-of-war, are, in their turn, the highest development of the swim- 

 ming bells. They are, in fact, colonies of bells, joined together in almost every 

 possible way, and showing extraordinary modifications of individuals in the 

 interests of a division of labor. For instance some of the bells do nothing but 

 row the colony along, others feed the colony, others are guards, and yet others 

 produce the eggs. As our first example of the group, we may take the creature 

 known as Physophora, which consists of a long tube or central axis, surmounted 

 by an individual which is nothing but an air vesicle for holding the colony in an 

 upright or a sloping position in the water. Below the air vesicle come two rows 

 of bells, which bring about by their contractions the movement of the whole 

 colony. These rowing bells force the water out of their cavities, and thus propel 



