ACTINOZOA. 149 



as destitute of distal orifices. The pinnse are very- 

 contractile, so as to vary in form from mere lobes 

 or tubercles to long filiform fringes. But little 

 diversity is exhibited by the tentacles of this order. 

 Except in the distinctive characters just mentioned, 

 they agree essentially with those of Actinia. 



The tentacles of the Ctenophora are best des- 

 cribed in connection with the general survey of 

 the characters of that order. 



5. Tegumentary Organs. — In but few Acti- 

 nozoa do the tentacles appear to be processes of 

 the ectoderm only. This layer, as we have seen 

 in Actinia, exhibits a tendency to differentiate into 

 two diverse planes of growth, which, with Professor 

 Huxley, we may designate the ' ecderon ' and the 

 ' enderon', respectively. Sometimes, however, 

 this distinction is not observable. The ectoderm is 

 usually ciliated, and in the Ctenophora becomes 

 very thick and gelatinous, presenting a structure 

 somewhat similar to that which occurs in the 

 oceanic Hydrozoa. G-egenbaur describes the re- 

 ticulating threads which traverse the periplastic 

 mass as tubular in young Ctenophora, but, as 

 growth advances, tending to become solid. Other 

 minor histological modifications have been ob- 

 served. 



The general surface of the body, smooth in most 

 Ctenophora, is in Chiajea, and a few other genera 

 diversified at intervals by the elevation of numer- 

 ous simple papillae. And, in some Sea-anemones, it 

 exhibits a number of clear warts or vesicles, each of 

 which, according to M. Hollard, possesses a muscu- 

 lar arrangement of its own, in connection with a 

 sort of two-lipped mouth; so that a needle, or 



l3 



