28 Coelenterata. 



absence of an aboral hydranth when the latter develops regardless of the 

 orientation of the piece (see Bericht f. 1903 Coel. p 17 Hazen). The aboral 

 pieces of the 20 polyps were inverted so that the cut oral surface came in con- 

 tact with the substratum. In every case but 1 the pieces righted themselves 

 and regeneration of hydrauths ensued. The single exception remained as it 

 was placed and developed over the cut end a smooth surface resembling a 

 foot, though it did not adhere, and the piece finally died. The longer pieces 

 developed heteromorphically much more readily than the shorter ones. 



Child (^ states that the inrolling of the margins and the closure of openings 

 (in the body-wall of Cerianthus) by contact of the inrolled margins is the result 

 of the elasticity of the body-wall. This elasticity must be greater in the 

 inner than in the outer portions to produce the observed results. The mesogloea 

 plays the most important part in this elastic contraction. Openings between 

 folds of the mrolled body-wall may be closed by ectoderrnal slime secretion; 

 this often occurs in pieces before the formation of new tissue and permits the 

 existence of considerable water-pressure in the ccelenterou. Contact or close 

 approximation between two cut surfaces, or parts of a cut surface, is a ne- 

 cessary condition of the growth of new tissue from these surfaces. A single 

 exposed cut surface may heal over, but no further growth occurs from it. The 

 new tissue having arisen at a point of contact between two cut surfaces is 

 capable of extending, in the form of a thin membrane between diverging cut 

 surfaces, to distances which depend on the angle of divergence of the cut sur- 

 faces and on the thickness and quality of the membrane. 



Child ( 2 ) finds that, except when the animal is contracted or collapsed, the 

 body-wall of Cerianthus is subjected to a certain amount of tension, the water 

 in the coelenteron being under pressure. This internal pressure is probably 

 established and maintained by the inward current along the siphonoglyph. When 

 the body is distended the stomodamm (except the siphonoglyph and perhaps some 

 grooves) must be closed. When contraction occurs the fluid from the coelenteron 

 first issues from the aboral pore (in other Actinians from the cinclides, etc.); 

 then, when the pressure is sufficiently reduced to permit it, the walls of the 

 stomodaeum are separated by muscular action and the remaining fluid issues 

 from the mouth, often accompanied by meseuterial filaments. The stomodseurn 

 is widely open only during extreme contraction. The cilia on the endodermal 

 surface of the body-wall produce a current flowing orally in each mesenterial 

 chamber. The internal water-pressure plays a large part in form regulation in 

 Cer. The general pressure affects the rapidity of growth wherever it may be 

 taking place, and possibly the local pressure exerted on the body-wall by the 

 currents passing orally in each mesenterial chamber is the formative stimulus 

 for the marginal tentacles. Regeneration of tentacles is impossible unless 

 mesenteries are present, in the absence of mesenteries there is no localisation 

 of the currents and the fluid in the coelenteron exerts less pressure on the in- 

 rolled oral end than if mesenteries are present. Local retardation or inhibition 

 of tentacle-regeneration can be brought about by preventing a distension of a 

 part or parts of the oral region. According to Child ( 3 ) in pieces of Ceri- 

 anthus which are kept open or opened repeatedly at the aboral end the ap- 

 pearance and growth of the tentacles, the growth of the disc and all other regene- 

 rative phenomena at the oral end are delayed. The marginal tentacles which 

 appear under these conditions are short and blunt. Labial tentacles rarely appear 

 until closure and distension have occurred, after which the regeneration resumes 

 its typical course. These experiments show the important influence of the 

 general internal water-pressure upon regeneration. Child ( 4 ) finds that in 



