The Medusm of Millepora. 



5 



ably in size, but the majority were about 0'4 mm. in diameter, and the 

 remainder rather smaller. The structure of these bodies was examined 

 by means of sections taken horizontally and vertically to the surface of 

 the corallum. Each medusa (fig. 1) lies in an ampulla (amp.) or cavity 

 in the corallum, and is attached by a narrow stalk to the centre of the 

 innermost wall of the ampulla. The umbrella (fig. 1, umb.) is a thin 



ElG. 1. 



^ mm 



membrane, slightly swollen at the margin (m.u.). With a high power 

 of the microscope it can be clearly seen that this membrane consists of 

 a median lamella of endoderm, covered on each side by an ectodermal 

 epithelium. No canals or cavities of any kind occur in the umbrella. 

 There is no velum, and there are no tentacles. The cavity of the 

 umbrella is almost completely filled with a swollen manubrium, which 

 bears on its outer or upper side a mouth (M), around which lies a 

 broad band of ova. There can be no doubt that in some cases there is 

 communication between the endodermal cavity of the medusa and the 

 sub-umbrella cavity by way of this mouth, but it is impossible to say in 

 the present state of the inquiry whether the mouth of the medusa opens 

 normally at this stage. It appears to be closed usually at the time 

 when the medusa is ready to escape, as will be mentioned later on. 

 The great size, however, of the manubrium of the female medusa is 

 principally due to the thick vacuolated endoderm (end.). The gastral 

 cavity of the medusa is not so simple as it is in the male medusa (of 

 the Pacific Millepores), but it is subject to many very striking varia- 

 tions. In some of the medusae there are four radial co3ca proceeding a 

 short distance into the endoderm of the manubrium from the main 

 tubular gastral cavity that occupies the axis of the manubrium. This 

 cavity communicates, on the one hand, with the exterior by the mouth. 



