THE EDWARDSIA-STAGE OF LEBEUN^IA. 297 



Such an occurrence in a Perforate coral, iu which the circu- 

 latory system is very extensive and complex, seems to afford 

 strong evidence for regarding the single filament as the homo- 

 logue of the whole of the trilobed filament, and consequently 

 for its origin as a whole from one and the same embryonic 

 layer. 



In the adult Lehnmia the ciliated streaks are strongly de- 

 veloped and continuous with the lining of the oesophagus at its 

 inner termination. 



IV. B,ELA.TIOKS or THE TeFTACLES AND MESENTERIES. 



The origin of the diff'erent tentacles in relation to the mesen- 

 terial chambers discloses some unexpected conditions. And firstly 

 a comparison may be instituted between the time of appearance 

 of the tentacles as external organs of the polyp, and the stage 

 reached in the development of the internal mesenteries and their 

 corresponding chambers. 



Though the first eight mesenteries in all Zoantharia yet 

 investigated develop successively in pairs, the early tentacles 

 rarely do so. Lacaze-Duthiers, in his classic investigations 

 already referred to, found the tentacles to appear in Actinia 

 equina during the stage in which eight mesenteries were present^ 

 one from each mesenterial chamber, as is also the case in 

 Lehrunia. Von Koch (1897) apparently found twelve tentacles 

 to appear simultaneously in Caryophyllia cyathus, at a stage when 

 twelve mesenteries were developed, eight only of which were 

 perfect. In Haddon's newly-hatched larvae of Eupliyllia no 

 tentacles, however, were developed, although six pairs of mesen- 

 teries were present, three pairs of which possessed filaments. 



On the other hand, both McMurrich and van Beneden found 

 that in the Cerianthid Arachnactis the appearance in pairs of the 

 tentacles follows closely upon that of the formation of the mesen- 

 terial chambers. But, in consequence of the stomodseum ex- 

 tending right across the ccelenteron, from wall to wall, at the 

 stage of development characterized by the presence of two, or 

 perhaps three, pairs of lateral tentacles, there exists no trace of 

 any median chambers, and it is only when these have been 

 formed that a median tentacle appears. In this case it is the 

 ventral tentacle ; a median dorsal never occurs, the region being 



