JELLYFISHES 135 
From the margin of the umbrella depend the tentacles. There 
are little mineral deposits, like crystals, called lithocysts, disposed 
at intervals on the margin, and known also as marginal bodies, 
which are supposed to be eyes. In some species these lithocysts 
are inclosed in club-shaped bodies, and they are then called ten- 
taculocysts, because they are like small tentacles. These, together 
with the nerve-fibers, are called the sense-organs; but to what 
extent jellyfishes can see and feel is undetermined. This is the 
first appearance of sense-organs in animals. Around the con- 
cave surface of the umbrella is a muscular zone, or zone of con- 
tractile tissue, by which the animal opens and shuts the umbrella 
and gets its locomotive power. The gonads, which are con- 
spicuous from being more opaque than the rest of the body, are 
the egg- or sperm-sacs. They vary in form and in position. 
The jellyfish is carnivorous, feeding on small organisms such 
as crustaceans and even fishes. The tentacles are invested with 
stinging-cells, as are also the frills about the mouth, when such 
occur. With these stinying-cells, which are in some species so 
powerful as to have been compared with an electric ‘battery, the 
jellyfish benumbs its prey. The stinging properties are due to 
nettle-like threads contained in poison-cells. When these pene- 
trate the flesh they produce a pain similar to that of an electric 
shock. 
The food is taken into the manubrium by the square mouth at 
its free end, and is there digested. It is then sent as nutritive 
fluid through the canal system of the body, and ejected through 
small pores in the canal which surrounds the margin of the 
umbrella. 
There are two sexes. The gonads of the female contain eggs; 
those of the male, sperms. The contents of the gonads drop 
into the central cavity and pass out through the mouth. The 
fertilized ovum is called a plana, and is a transparent sphere 
covered with cilia, by means of which it swims about for a time. 
At length it attaches itself to some object, and becomes in some 
species a branching colony (hydroid), in other species a strobila. 
The latter, as it grows, is constricted at intervals, and at maturity 
resembles a pile of inverted saucers with lobed edges. Hach of 
