AFFINITIES OF OLHENULPHORES. 93 
Ctenophora, as, for example, that of Beroé, agrees in general 
with that of the jelly-fishes, with the difference that in the 
Ctenophores the nerve-centres are not situated on the edge, 
but at the pole of the body opposite the mouth. On the 
other hand, the nervous system is not radiated as in the 
jelly-fishes or as in the Echinoderms. 
Our commonest example of this class is the Plewrobrachia 
rhododactyla Agassiz. It is a beautiful animated ball of 
transparent 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 thousands may be seen 
stranded on the shore at low water. 
That the Ctenophores have affinities to the sea-anemones 
(Actinozoa) is seen in the form and relations of the diges- 
tive tract, though it differs in hanging free, not being held 
in place by radiating mesenteries, and in this respect they 
approach the Echinoderms. From their possessing a dis- 
tinct digestive tract, the Ctenophores need not be confounded 
with the jelly-fishes (Hydrozoa). On the other hand, they 
present some advance over the Actinozoa, and in some 
respects connect the Hydrozoa and <Actinozoa with the 
Echinoderms. For example, the water-vascular system 
arises in the Ctenophores as outgrowths from the digestive 
sac, as they do in the young star-fish and sea-urchins. This 
indicates that in the mode of development of both the di- 
gestive tract and the water-vascular system the Ctenophores 
are allied to the Echinoderms rather than to the Hydrozoa, 
in which the water-vascular tubes arise as simple hollows in 
the body-mass. Moreover, they are less radiated than in 
the Hydrozoa or Echinoderms. 
In Bolina alata Agassiz the body is plainly bilateral and 
the water-vascular tubes are very distinct. In Idyia roseola 
Agassiz 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 Oteno- 
phores, however, even their eggs and embryos, are phospho- 
rescent, In the Ctenophores the ovaries and spermaries occur 
in the same individual and form blind sacs attached to the 
