FORM-REGULATION IN CERIANTHUS. 1 49 



of growth and size of tentacles. The larger tentacles belong 

 to those chambers which happened to be more or less widely 

 open during the period of collapse. Some of the tentacles did 

 not appear at all until complete closure and distension had oc- 

 curred. In all cases after closure and distension the irregularities 

 were gradually reduced until an approximation to the typical 

 condition was attained. It seems scarcely necessary to multiply 

 detai's further. My observations and experiments have led me 

 to believe that in every case of this kind it can be shown that 

 absence of tentacles or delay in their appearance is correlated 

 with contraction, folding, etc., which may prevent the occurrence 

 of circulatory currents in the intermesenterial chambers or reduce 

 their force and volume. 



The form of the marginal tentacles in open pieces is usually 

 markedly different from that of typical tentacles, the former being 

 blunt and rounded in form, often almost sac-like, instead of 

 slender and tapering. A comparison of Figs. 2 and 4 with Figs. 

 3 and 5 affords an illustration of these differences ; Fig. 16 

 represents an extreme case of this kind. In all such cases the 

 blunt tentacles increase in length and soon acquire an almost or 

 quite normal form after closure occurs. This peculiar form of 

 the tentacles in open pieces may be regarded as due indirectly to 

 the reduced size of the margin of the disc in these more or less 

 collapsed pieces (cf. Figs. 2 and 4 with 3 and 5). Since the 

 margin of the disc is reduced the opening into the tentacle from 

 the intermesenterial chamber is smaller than in distended pieces, 

 consequently the movement of water through it is impeded. 

 Thus the local pressure exerted by the circulatory currents must 

 be to a large extent eliminated within the tentacles, or at any 

 rate insufficient to cause continued increase in their length. The 

 general internal pressure, however, must be the same in the 

 tentacular cavity as in the mesenterial chambers and it is possible 

 that this has some effect upon the thin new tissue of the tentacles 

 in causing enlargement in all directions. 



It is possible also that the form of the tentacle may be affected 

 to a greater or less extent by the rate of growth. In the open 

 pieces the rate of growth is always slower than in closed pieces 

 in consequence of the lower pressure in the former. Material 



