ACTINOZOA. 149 



as destitute of distal orifices. The pinnae 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 Ctenojphova are best des- 

 cribed in connection with the general survey of 

 the characters of that order. 



5. Tesiinientary Or«;ans. — In but few Acti- 

 nozoa do the tentacles appear to be processes of 

 the ectoderm only. This layer, as we have seen 

 in J. c^/?iia, 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. Gregenbaur describes the re- 

 ticulating threads which traverse the periplastic 

 mass as tubular in young Ctenophora, but, as 

 groT\i:h 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 papilloe. 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 

 L 3 



