FORM REGULATION IN CERIANTHUS. 285 



sion in consequence of the fact that the water in the enteron is 

 under pressure. 



2. The inward current along the siphonoglyphe is probably 

 the means by which the internal pressure is established and main- 

 tained. 



3. When the body is distended the oesophagus, with the ex- 

 ception of the siphonoglyphe and perhaps some grooves and 

 crevices, must be closed. 



4. When contraction occurs the water first issues from the 

 aboral pore (in other actinians from the cinclides, etc.); then when 

 the pressure is sufficiently reduced to permit it, the cesophageal 

 walls are separated by muscular action and the remaining water 

 issues from the mouth, often accompanied by mesenterial fila- 

 ments. Thus the oesophagus is widely open only during ex- 

 treme contraction. 



5. The cilia on the entodermal surface of the body- wall pro- 

 duce a current flowing orally in each mesenterial chamber. The 

 water passes from each chamber along the aboral face of the 

 marginal tentacle, back on its oral face beneath the disc toward 

 the stomodaeum, probably into and out of the labial tentacles and 

 aborally along the stomodaeum. In all probability cilia along 

 the sides or margins of the mesenteries force it further aborally. 



6. The internal water-pressure plays a large part in form-regu- 

 lation in Cerianthus. The general pressure affects the rapidity of 

 growth wherever it may be taking place and it is possible that 

 the local pressure exerted on the body-wall by the currents pass- 

 ing orally in each mesenterial chamber is the formative stimulus 

 for the marginal tentacles. 



7. Regeneration of tentacles is impossible unless mesenteries 

 are present. The reason suggested for this is that in the absence 

 of mesenteries there is no localization of the currents correspond- 

 ing to the intermesenterial chambers, and, moreover, the water 

 being unconfined between mesenteries, exerts less pressure on 

 the inrolled oral end than if mesenteries were present. 



8. Local retardation or inhibition of tentacle-regeneration can 

 be brought about by preventing distension of a part or parts of 



the oral region. 



HULL ZOOLOGICAL LABORATORY, 



UNIVERSITY OF CHICAGO, September. 1903 



