ON THE DEVELOPMENT OF TUBULIPORA. 77 



lamina, and grows at first in a more or less horizontal, centri- 

 fugal direction. As its length is added to at the growing 

 edge, its upper wall gradually rises above the general level of 

 the colony, and some of the zooecia thus form projecting ridges 

 (fig. 1). As the zooecium rises up in this way, its basal wall 

 splits ofi' from the basal lamina, between which and itself a 

 cavity thus originates. This cavity is the beginning of a new 

 zooecium, which continues to grow in the same manner. 



It follows from this description that each zooecium proxi- 

 mally reaches the basal lamina, as in Lichenopora (16, wood- 

 cut, p. 84); a younger zooecium always originating from the 

 basal side of an older one, and making its appearance distally 

 to it after a certain time. 



As the zooecium continues to grow longer, its distal part be- 

 comes free from the common basal mass of the colony. It may 

 become completely free on all sides, and it then, by the activity 

 of its own terminal membrane, develops into a curved tube, 

 which is generally cylindrical, though in some cases with an 

 oval transverse section and orifice. Very commonly in Tu- 

 bulipora the zooecium does not become free on its basal 

 side, where it remains connected with one or more of its 

 younger neighbours. In this way it forms the beginning of a 

 row, usually uniserial, of connate zooecia, as show in figs, 1, 5, 

 and 9. The adjacent zooecia of a series are separated by a 

 flat septum, corresponding with the intersecting plane of two 

 cylinders. Any one of the middle zooecia of a simple series 

 will thus have four flat walls (fig. 1). 



The shape of the orifice corresponds with that of the 

 zooecium, being at first angular in the case of serial zooecia. 

 If, however, growth of these goes on actively, the series may 

 be resolved above into its constituent units, each of which then 

 becomes cylindrical with a round orifice. 



The ovicell of Tubulipora is an enlarged zooecium. This 

 is shown by several facts, the most striking of which is that 

 when young it has a polypide, and then diff'ers in no way from 

 an ordinary zooecium. The proximal end of the ovicell thus 

 takes part in the formation of the basal lamina. On looking 



