ACTINOZOA. 165 



several genera. The surface of the ctenophore is 

 transversely elevated at intervals throughout the 

 greater portion of its length into a number of 

 successive ridges, to each of which a row of strong 

 cilia is attached in such a manner as to form a 

 paddle-like plate, or comb, the free extremities of 

 the cilia remaining separate. The cilia are not all 

 of equal length, those of the middle portion of 

 the comb usually having the advantage in this re- 

 spect, while the cilia on either side symmetrically 

 correspond ; their degree of elongation varying 

 so as to impart to the edge of the entire comb a 

 gently curved outline, when seen at rest. This is, 

 indeed, seldom the case during the life of the 

 animal, throughout which the combs manifest an 

 astonishing amount both of simultaneous and 

 successive activity. Nay, even after death, de- 

 tached portions of these creatures, bearing frag- 

 ments of the ctenophores, exhibit for many hours 

 no apparent diminution of their ordinary vibratile 

 efficiency. 



8. HVervous Slystem and Organs of liense. 



— In no ActinozocCf save the Ctenophora, has 

 good evidence of the presence of a nervous system 

 or organs of sense yet been obtained. Nor should 

 this appear surprising, for the sensitivity which, 

 in more highly differentiated organisms, has its 

 course restricted to definite tracks, is here diffused, 

 in a less appreciable manner, through the more 

 general and comparatively ill-developed tissues 

 of the body. The white or blue marginal sacs of 

 some Actinias, and the body-warts in allied species, 

 have, it is true, been regarded as sensitive in 

 function, and the former have even been dignified 



M 3 



