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 {Comh-learers). 
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 Pleurohrachia 
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 alafa the body is plainly bi- 
lateral and the water- vascular tubes are 
very distinct. In Idyia roseola (Fig. 36) 
the mouth is large, the stomach wide, and 
the body is of an intense roseate hue. 
This beautiful species after death, late 
in 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.), and 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. 
Fig. ZQ— Idyia rose- 
ola, natural size, 
a, anal opening: 6, 
lateral canal; c, cir- 
cular canal; d. e. /, 
g, h, rows of paddles. 
