JELLY-FISH, ETC. 
481 
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, Ccelenterates, 
muscle and nerve elements occur below, and are associated with, the stinging- 
cells. The action of these 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the animal knocks against a stone, or when the tentacles, being withdrawn, touch 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 towards 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 Jelly-Fish axd their Allies, —Class Polypomedusae. 
Order Siphonophora. 
We have already described the swimming-bells of the jelly-fish 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 
swimming-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 labour. 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 
the colony. Below these, again, comes a circle of extremely mobile tentacles, 
which may perhaps be the tentacles of vanished bells. Among these tentacles 
are hollow structures, open at the end, which are the feeding-bells, now reduced 
to sucking-tubes, or stomachs, each of which endeavours to seize and digest for 
itself whatever in the shape of food (chiefly small crustaceans) is brought to it by 
the long capturing filaments and their branches, armed with stinging organs. 
The colourless blood and nutritive fluid prepared by these two stomachs serve 
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