ON THE DEVELOPMENT OP TUBULIPOEA. 75 



fig, 2) formed by tlie calcification of the body-wall of the 

 metamorphosed larva. The early zoarial development is well 

 described by Barrois (1, p. 70, pi. iv, fig.), whose account 

 perhaps refers partly to T. phalangea and partly to T. 

 plumosa; and it results in all species in the formation of a 

 colony which is at first pyriform or fan-shaped. The terms 

 "proximal^' and " distaF' will be used with relation to the 

 primitive disc. The distal curved margin of the colony is 

 bounded by the 'terminal membrane" (16, p. 93), which 

 consists partly of an uncalcified cuticle (ectocyst) and partly of 

 underlying protoplasmic structures. The terminal membrane 

 does not remain as a continuous sheet, since portions of it are 

 continually cut off by the upgrowth of the calcareous septa 

 which form the lateral walls of the zooecia or ovicells. Each 

 of these structures is thus closed by a derivative of the original 

 terminal membrane, and this name may consequently be ap- 

 plied also to the uncalcified distal wall of each unit of the 

 colony (fig. 24). Both the zooecia and the ovicells are added 

 to, after their first formation at the growing edge, only by the 

 prolongation of parts of the calcareous tubes which have 

 already been developed, though this is not altogether true of 

 the Lichenoporidse, in which the character of the colony is 

 altered by the subsequent development of '' caucelli." It thus 

 follows that a young Tubulipora colony is identical^ so far 

 as its calcareous structures are concerned, with the proximal 

 part of the same colony at a later stage of its existence ; and 

 that in order to understand what a particular colony or ovicell 

 was like at an early period of its development, it is only neces- 

 sary to imagine the distal parts of the colony suppressed. 



The form of a particular colony is due to the behaviour of 

 its terminal membrane, which is identical in its extent with 

 the growing margin. Should this remain undivided, and con- 

 tinue to grow symmetrically, a pyriform or flabelliform colony 

 will result, according to the relative activity of growth in the 

 longitudiual and transverse directions. Under certain circum- 

 stances, probably due to unfavourable conditions, the colony 

 becomes mature by developing an ovicell without losing its 



