186 SYDNEY J. HICKSON. 



In the last stage I have observed the medusa is completely 

 separated from the canal system, and lies freely within the 

 cavity of the ampulla. The walls of the umbrella, except at 

 the margin, are extremely thin. The manubrial endoderm 

 contains a closed cavity (fig. 11). This stage is probably the 

 last that occurs before the embryo escapes from the corallum. 

 There are no nematocysts developed on the thickened margin 

 of the umbrella, there are no sensory bodies, there is no velum, 

 and no mouth. 



2. In the development of the medusae of Millepora that 

 has just been described there is no structure formed at any 

 time that can be compared with the inner fold of ectoderm or 

 " glockenkern " that forms the walls of the sub-umbrella cavity 

 of the medusa of the Hydroidea. Had such a structixre been 

 found, there might have been some ground for supposing that 

 this medusa is a bud that grows out of the degenerated tissues 

 of a zooid. The medusas of Millepora are, however, certainly 

 not formed by budding from the zooid in the sense that the 

 medusae of such a form as Corymorpha are budded from the 

 hydranth. 



3. The diagrammatic figures that are frequently given of 

 zooids of Millepora, representing a centrally placed gastro- 

 zooid in a complete circle of dactylozooids, is perfectly correct 

 for some species of Millepora and the younger branches of 

 others. 



In M. miirrayi the zooids are scattered over the older 

 parts of the corallum in an irregular manner. The circular 

 systems can be made out, but over and above the zooids in 

 their regular circles there are both gastrozooids and dactylo- 

 zooids scattered irregularly within and between the circles 

 (cf Quelch [11], p. 192). 



The medusa; occur both in the regular circles and irregu- 

 larly between them, as may be seen by reference to Woodcut 1. 

 "When a piece of Millepora is decalcified and cleared in oil of 

 cloves or turpentine, and examined with a low power of the 

 microscope, the arrangement of the zooids, medusae, and coeno- 

 sarcal canals can be very readily observed. The figure I have 



