CLOSUEE or THE CTCLOSTOMATOUS BRTOZOA. 403 



that one zooecial tube has a double closure (fig. 6), as mentioned 

 aboTO ; and the same thing may be seen in a Beticulipora fossil 

 from Aldinga, Australia, which I call _5. transennata. 



In a JRadiopora from St. Croix these divisions, instead of 

 being irregularly placed as in S. conifera, occur at longer in- 

 tervals, and then in the neighbouring zooecia they are frequently 

 at an equal distance from the centre of growth, thus forming, 

 when slightly magnified, almost concentric lines cutting across a 

 number of zooecia. The section of Liclienopora holetiformis shows 

 the structure of an Australian fossil, which in its shape and 

 general appearance partly resembles Badiopora ; but the way 

 in which it grows will be seen to be quite different. If we turn 

 to Simonowitsch's ' Beit. z. Kennt. der Bryozoen des Essener 

 Grriinsandes,' I think we see both structures ; for in fig. 4, 

 pi. i., we seem to have a Badiopora, while figs. 2 and 3, pi. ii., 

 seem to represent a Lichenopora growing like L. cocJiloidea, 

 d'Orb., or L. holetiformis, d'Orb. j and in such cases there is 

 probably a basal lamina as in the Australian fossil. 



It certainly does not seem that we can in any way directly 

 compare this immovable closure with the movable operculum of 

 the Chilostomata ; but at the same time the object may not be 

 entirely dissimilar. To understand this, we must always keep 

 in mind that with the death of the polypide, or perhaps it is 

 better to say the absorption, the colony continues its life, and in 

 the case of Chilostomata a new polypide may grow in the zooecium 

 recently occupied ; but to what extent this is the case in Cyclo- 

 stomata I do not find any direct observations. Now if each 

 zocecium during its polypideless condition could be choked up by 

 the mud deposited from the sea, then the whole colony must 

 sufier from the absence of the individual polypides which kept the 

 living tissue free, and the death of the colony would result. 



In the Cyclostomata, which are the simplest of the Bryozoa, 

 a calcareous partition exists which closes the tubular zooecium, 

 and thus protects the colony ; whereas in the Chilostomata, which 

 are more highly differentiated and, not being tubular, could more 

 easily be closed up, there is the horny operculum, which is not a 

 sign of death, but is movable, and protects the living polypide 

 and through it the colony. I have already carried this idea 

 further and applied it to avicularia (Quart. Journ. Geol. Soc. 

 vol. xxxvii. p. 327, and vol. xxxix. p. 425) ; for the avicularia 

 retain their life and power of movement when no polypide fills 



