36 



ZOOLOQT. 



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. — Ctenophoea {Coml-hearers), 



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 body, with two posterior outlets. 



Our commonest example of this class is the PIcurohracMa 

 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 body two long ciliated tentacles. It 

 is abundant in autumn; sometimes thou- 

 sands may be seen stranded on the shore 

 at low water. 



In Bolina (data the body is plainly bi- 

 lateral and the water- vascular tubes are 

 very distinct. In Idijia roseola (Fig. 3G) 

 Fig. ze.—TrJm'n. ro.ie- the mouth is large, the stomach wide, and 



o?ri, natural size, i i t • n "^ - 



o, anni opening:/), the bouy IS of an intense roseate hue. 

 cuiar canal; 'ri.'p. f, This beautiful species after death, late 



ff./t, rows of paddles. • _ • , , , ii 



in summer, is very phosphorescent; all 

 Ctenophores, however, even their eggs and embrj'os, are 

 phosphorescent. 



* See Semper's Aniiiml J.ile, A. Agas.siz's Three Cruises^oTthe 

 Bluke (vol. i.), and tlie works of others who deny the theory of Dar- 

 win and of Dana, that suljsidence i.s 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 ilie formation 

 of reefs over cue hundred feet tlijcls. 



