JELLYFISHES 135 



From the margin of the umbrella depend the tentacles. There 

 are little mineral deposits, like crystals, called Uthocysts, 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- 

 tacidocysts, 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 stinging-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 planula, 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 stroNla. 

 The latter, as it grows, is constricted at intervals, and at maturity 

 resembles a pile of inverted saucers with lobed edges. Each of 



