112 SIDNEY ¥. HARMEK. 



Nitsche in Membranipora membranacea. The vestibule 

 is lined with a flexible cuticle, which can be made out in 

 longitudinal sections through an adult orifice as a collapsed 

 tube continuous externally with the chitinous part of the ter- 

 minal membrane of the zooecium, and internally with the dia- 

 phragm, through which the tentacles are protruded. 



In a living, perfectly healthy colony, each zooecium is seen 

 to be closed by a delicate terminal membrane, perforated by a 

 minute hole at its centre, the membrane and its perforation 

 being stretched across the extreme end of the calcareous 

 zooecium. A similar membrane closes the young ovicells, and 

 appears to be stretched continuously over the edges of the 

 calcareous septa which are forming the walls of new zocecia at 

 the growing margin of the colony. The size of the perforation 

 in the membrane of the adult zooecium can be altered in a way 

 which suggests the alterations in the diameter of the pupil of 

 the eye, though I have not succeeded in demonstrating the 

 mechanism of the movements. Before protrusion of the ten- 

 tacles the pupil, or in reality the opening of the introvert, is 

 widely dilated, and the tentacles are pushed through it. 



The perforated terminal membrane appears to be similar to 

 what JuUien (21, p. 38) has described as the '^irisoidea" in 

 Cheilostomes (Microporella malusii), although Pergens 

 (36, p. 509) has adversely criticised JuUien's results on this point. 



In colonies which are still alive, but less healthy, the ter- 

 minal membrane may no longer appear flush with the surface 

 of the zooecia, but may be sunk down to some little distance 

 from the orifice (fig. 29), and be deeply concave distally or 

 upwards. There is little doubt that this is the result of an 

 unhealthy condition; but it is important to notice that the 

 position of the terminal membrane can vary in the tube be- 

 cause in sections and in colonies which have been mounted 

 whole the distal ends of the zooscia are often completely empty 

 of cellular structures for a distance equal to once or twice the 

 diameter of the zooecium. The same phenomenon is noticed 

 in the ovicells, in which the growing edge of the living body- 

 wall is usually at some distance from the edge of the cal- 



