THE GONOPHORES OF THE HYDROCORALLINiE. 197 



former there is a lumen and in the case of the latter the endo- 

 derm is solid. 



In comparing the structure of the phanerocodonic medusa 

 and the medusa of Millepora a very general similarity may be 

 observed. 



In both there is a centrally placed manubrium (a). 



In both the generative elements (b) are developed between 

 the ectoderm and endoderm of the manubrium. 



In both there is a contractile bell umbrella, from the centre 

 of whose concavity the manubrium is suspended ; in both this 

 umbrella is composed of a centrally placed sheath of endoderm 

 covered by a sheath of ectoderm on both sides ; and in both 

 the gonophore lies in a gonangial cavity of ectoderm, which, 

 before the medusa is set free, is continuous with the ectoderm 

 of the outer wall of the umbrella. 



The principal points in which these two forms differ from 

 one another are these : 



The manubrium of 2 possesses a mouth. 



The manubrium of 3 does not. 



There is a system of canals (longitudinal and ring) in the 

 umbrella of 2. 



There are no canals in the umbrella of 3. 



There are tentacles and sensory epithelium at the margin of 

 the umbrella in 2. 



There are no tentacles or sensory epithelium at the margin 

 of the umbrella of 3. 



There is a velum in 2. 



There is no velum in 3. 



Too much stress should not be laid upon any of these points 

 of difference, for it is quite possible that tentacles, eyes, or 

 auditory organs, a velum and a system of gastro- vascular canals, 

 may be subsequently developed in the medusa of Millepora after 

 it is set free. 



It is of importance to note, however, that these organs are 

 not developed while the medusa is still attached to the parent 

 stock, as they are in the typical phanerocodonic medusa of the 

 Tubulariffi. 



7 



