THE MEDUSA. 



simple or ramified, carry the digested food to the circular canal. No special organs of circulation 

 exist, and respiration is effected by the membranes or skin of the disc. 



The fringe of tentacles around the disc may be very short and sparely developed, or these 

 appendages may be many feet in length and very numerous. They are supplied with nernatocysts, 

 which are the stinging organs, and which are sufficiently annoying to some thin-skinned bathers. 



The nervous system may exist in relation to the eye-spots, and in a very rudimentary condition 

 elsewhere. Small swimming Invertebrata are the food of the Medusa. 



The methods of reproduction and development are very remarkable, and the dimensions of the 

 full-grown disc are greatly in ^^ - _ ^__ ^_ = ^ _ __^ -__- J 



excess of those of the first stage 

 of life. One great group of the 

 Discophorse, including the com- 

 mon Jelly-fish of the British \ 

 seas, lay eggs in the autumn 

 when they are swimming near 

 to the coasts and estuaries. 

 The parent dies, and the young 

 escape from the eggs as little 

 spherical bodies, covered with 

 cilia. Each one attaches itself 

 by its base to a rock or seaweed, 

 and tentacles are formed at the 

 other end, the body gradually 

 becoming elongate. With 

 growth some contractions occur 

 around the young form, the 

 first being just below the circle 

 of tentacles. Tentacles soon 

 appear on the edges of the con- 

 traction nearest the base, and 



the edges of the other con- 

 tractions simply become lobed. 

 After a while these contractions become deep, and the animal resembles a set of plates placed one 

 over the other, the top and bottom ones having circlets of tentacles. At a certain period, when the 

 whole is less than an inch in height, the entire structure breaks up ; the top falls off and dies, and 

 the bottom part remains fixed, whilst the rest separates into as many discs as there were contractions, 

 and each swims off to become a gigantic Discophora. * This process is a good 

 example of the alternation of generation, and the young and tentacled form is 

 the nurse or intermediate stage. It has been called " Hydra-tuba," and in the 

 next stage it is called Strobila. 



The Discophora? include the great free-swimming oceanic Medusae, but it 

 is not clearly proved that all do pass through the peculiar stages of develop- 

 ment. Some may have a very different early life, and may belong to other 

 groups of the Hydrozoa. Two sub-orders are distinguished. The Pelagida, 

 containing such genera as Cyanea, Aurelia, and Pelagia, have a large central 

 mouth surrounded with four arms, often subdivided, and fixed on to a buccal 

 peduncle. The fringes of tentacles on the lobed margin of the disc may be long 

 or short. The genital organs are four. The Rhizostoma have a great develop- 

 ment of the structures surrounding the mouth, which gives the name from 

 their root-like appearance. The mouth, with growth, closes at its lips, and 

 passages into the digestive tract are formed down the rootlets, at the ends of which are small 

 openings, like little suckers. There are no marginal filaments. 



* Example, Aurelia flavidula. 



Al'UELIA AfHITA. 



STKOB1LA OF AURELIA 



FI.AVIUULA. 

 (After L. Agassiz.) 



274 



