SECT. in. ARMED OR STINGING MEDUSAE. 99 



these armed tentacles, which are very numerous in the 

 covered-eyed group, the gelatinous umbrella has a mul- 

 titude of oval thread-cells on its external coat, in each 

 of which a very long filament is spirally coiled, which 

 darts out to a considerable distance on the smallest 

 touch, and stings severely. 



A few only of the British pulnionigrade medusae sting : 

 the Cyanea capillata, one of the single-mouthed covered- 

 eyed family, is most formidable. It has very long ten- 

 tacles, which it can throw off if they get entangled, but 

 they continue to sting, even after they are detached from 

 the medusa. 



This is one of the most remarkable instances of the 

 inherent irritability of muscular fibre still in full force 

 after the tentacles have been separated from the living 

 animal. In many of the lower animals, as in the Hydra 

 itself, vitality is so far from being extinguished in the 

 severed members that it repairs the injury. Since the 

 covered-eyed medusae have eyes, ears, and very sensitive 

 tentacles, it may be inferred that they possess nerves of 

 sight, hearing, and touch, though none have been dis- 

 covered, probably on account of the softness and trans- 

 parency of their tissues. The stinging power by which 

 they kill their prey and defend themselves may be 

 classed among the consensual powers prompted by the 

 sympathetic sensations of hunger or danger. 



In all latitudes the medusae are highly luminous, espe- 

 cially in warm seas. Professor Yogt remarked that 

 flashes of light passed over their disk when they touched 

 one another in swimming, and they appear at intervals 

 like globes of fire among the lesser lights of the Nocti- 

 lucaa ; if from involuntary nervous contraction, as is 

 most likely, the light must be electric. 



The medusae are infested by many parasites. Entozoa 

 are often abundant in their gelatinous substance, and 

 crustaceans of various kinds and colours, such as shrimps, 



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