FORM-REGULATION IN CERIANTHUS. 



99 



with the other reduced tentacles is due to the greater degree of 

 distension of the intermesenterial chambers below them. It may 

 be that these chambers are not completely shut off from the 

 enteric cavity or possibly the pressure of the water in the dis- 

 tended region causes filtration of water into these two chambers. 

 Figs. 4 and 5 illustrate very clearly the relation between char- 

 acteristic form and internal water pressure. 



November 20. Twenty seven days after section (Fig. 6). 



FIG. 5- 



Terminal marginal tentacles on uncut side 8-9 mm.; on cut side 

 barely visible 0.25 mm.: lateral marginal tentacles 8-9 mm. 

 Terminal labial tentacles on uncut side 2 mm., on cut side ab- 

 sent : lateral labial tentacles 2 mm. The portion above the 

 lateral cut is much shrivelled while all other parts are fully dis- 

 tended and in good condition. The lateral and terminal discs 

 are now at almost the same level (Fig. 6), the part which origin- 



