3 I 2 COELENTERATA SCYPHOZOA 



, 



hundred young horse-mackerel (Caranx trachurus) may be found 

 sheltering under the umbrella of Rhizo stoma pulmo. As the animal 

 floats through the water the little fishes hover round the margin, 

 but on the slightest alarm dart into the sub-umbrella cavity, 

 and ultimately seek shelter in the sub-genital pits. 1 



Two species of fish accompany the American Medusa Dactylo- 

 metra lactea, one a Clupeoid, the other the young of the 

 Butter-fish (Stromateus triacantlius). According to Agassiz and 

 Mayer 2 this is not an ordinary case of mutualism, as the fish 

 will tear off and devour fragments of the tentacles and fringe of 

 the Medusa, whilst the Medusa will in its turn occasionally 

 capture and devour one of the fish. 



A great many of the Scyphozoa, particularly the larger kinds, 

 have the reputation of being able to sting the human skin, and 

 in consequence the name Acalephae 3 was formerly used to 

 designate the order. Of the British species Aurelia aurita is 

 almost harmless, and so is the rarer Rhizostoma pulmo ; but the 

 nematocysts on the tentacles of Cyanaea, Chrysaora, and Pelagia 

 can inflict stings on the more delicate parts of the skin which 

 are very painful for several hours, although the pain has been 

 undoubtedly greatly exaggerated in many popular works. 



The soft structure of the Medusae does not favour their pre- 

 servation in the rocks, but the impressions .left by several genera, 

 all belonging apparently to the Ehizostomata, have been found 

 in Cambrian, Liassic, and Cretaceous deposits. 



There is reason to believe that many Scyphozoa exhibit a con- 

 siderable range of variation in the symmetry of the most important 

 organs of the body. Very little information is, however, at 

 hand concerning the variation of any species except Aurelia 

 aurita, which has been the subject of several investigations. 

 Browne 4 has found that in a local race of this species about 

 20 per cent exhibit variations from the normal in the number of 

 the statorhabs, and about 2 per cent in the number of gastric 

 pouches. 



The Scyphozoa are not usually regarded as of any commercial 

 or other value, but in China and Japan two species of Ehizosto- 

 mata (Rhopile?na esculenta and R. verrucosa) are used as food. 



1 F. W. Gamble. See E. T. Browne, Proc. Boy. Irish Acad. 1900, p. 735. 



2 Bull. Mils. Co7np. Zool. xxxii. 1, 1898. 



3 d/ca\7?077 = a nettle. 4 Biometrika, i. 1901, p. 90. 



