36 ZOOLOGY. 



fathoms below the surface, yet owing to the slow sinking 

 of the island, they build up the reef as rapidly as the 

 former subsides, and in this way after many centuries a 

 coral reef sometimes two thousand feet thick may be built 

 up in mid-ocean. 



Without doubt ocean currents modify the forms of coral 

 islands and reefs, and have much to do with their arrange- 

 ment and distribution.* 



Class III. — Ctenophora (Comb-bearers). 



General Characters of Ctenophores. — These beautiful ani- 

 mals derive their name from the vertical rows of comb-like 

 paddles (ctenophores), situated on meridional bands of 

 muscles which serve as locomotive organs. Their digestive 

 tract passes through the bod}', with two posterior outlets. 



Our commonest example of this class is the PUurobrachia 

 rhododacyla. It is a beautiful animated ball of transpa- 

 rent jelly moving through the water by 

 means of eight rows of minute paddles, 

 throwing out from a sac on each side of 

 the bodv two long ciliated tentacles. It 

 is abundant in autumn; sometimes thou- 

 y sands may be seen stranded on the shore 

 at low water. 



In Bolina alata the body is plainly bi- 

 lateral and the water-vascular tubes are 

 very distinct. In Idyia roseola (Fig. 36) 

 fig. 86.— idyia roue- the month is large, the stomach wide, and 



via, natural size. ,•,■,-,.»., i 



a, anal opening: p. the body is of an intense roseate hue. 



cuiaTcanai; 'tf.'^f, This beautiful species after death, late 

 <,,>*, rows of pad.ues. jn summer> is very phosphorescent ; all 



Ctenophores, however, even their eggs and embryos, are 

 phosphorescent. 



* See Semper's Animal Life, A. Agassiz's Three Cruises of the 

 Blake (vol. i ). aud the works of others who deny the theory of Dar- 

 win and of Dana, that subsidence is necessary to account for the for- 

 mation of atolls, and claim that they are due to ocean currents, 

 wave action, etc., subsidence only being necessary in the formation 

 of reefs over one hundred feet thick. 



