THE SPONGES AND THE TWO-LAYERED ANIMALS 



153 



Fig. 8-16. A giant jellyfish (Cyanea capillafa) common on our eastern seaboard. The dome is flattened during 

 relaxation; during its power stroke it becomes more oval in shape. 



(sensitive to light) and statocysts (sensi- 

 tive to gravity), are located at the edge 

 of the bell where the nerve net is some- 

 what centralized. The statocysts are hollow 

 spheres containing small calcareous gran- 

 ules which, as they tumble about in the 

 cyst, stimulate nerve endings and indicate 

 to the animal its relation to the rest of the 

 world. In other words, it functions as an 

 organ of equilibrium, much the same as 

 the semicircular canals in our ears. This is 

 the first appearance of such an organ in the 

 animal kingdom. 



Sometimes, following a strong in-shore 

 breeze, thousands of large jellyfish are 

 swept up on the beaches to perish. Al- 

 though they are thought to be very jelly- 

 like, they do maintain their shape out of 



water and seem almost semi-solid. They 

 also retain some of their delicate beauty. 

 When they are swimming in the open sea, 

 they provide a sight that, once seen, is not 

 easily forgotten. 



Representatives of the next class, Antho- 

 zoa, are characterized by a heavy body, 

 supported by numerous septa, transverse 

 sheets of tough tissue. Members of this 

 group possess no medusa stage, eggs and 

 sperms being produced within the body 

 and discharged into the surrounding sea. 

 The most common representatives are the 

 sea anemones and corals; others less com- 

 mon are the horny, black, and soft corals, 

 the sea pens, and sea pansies. They com- 

 pare favorably with the jellyfish in respect 

 to beauty and numbers. Both the Atlantic 



