AFFINITIES OF CTENOPHORKS. 93 



Ctenophora, as, for example, that of Beroe, 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 Pleurobrachia 

 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 Hydrqzoa, 

 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 Cteno- 

 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 



