HYDROZOA. 



55 



from the parent colony and swam away. In the case of many Case 3, 



MedusiB, the eggs develop directly into Medusas, no fixed stage Upnght part. 



being known ; here the fixed stage, which there is reason to believe 



has at one time existed in every case, has been " hurried through " 



in the egg. Medusai present two very different types of structure : 



in one, which may be called the " Hydromedusan " type, the 



umbrella is provided with a velum, the sense-organs are naked on 



the umbrella margin (Naked-Eyed Meduste), and the fixed stock, 



when known, is of the Hydra or Hydroid type, and produces buds 



laterally. Medusee of this type come under the sub- class Hydro- 



medfisce, which includes Hydroida and HydrocorallincB. 



In Mcdusie of the second or " Scyphomedusan " type the 



Fig. 17.1 



Female Medusa of Millepora, showing the eggs and marginal batteries 

 of thread cells. (After Prof. S. J. Hicksou.) 



umbrella is without a velum, the marginal sense-organs are covered 

 by little lappets (Covered-Eyed Medusae), rows of " gastral fila- 

 ments " project into the stomach cavity, and the fixed stock, only 

 known in a few cases, is a broad flattened polyp (Scyphistoma 2), 

 which produces buds by transverse fission (Fig. 19a-c), and has four Case 3, 

 longitudinal ridges along the walls of the stomach cavity. Scypho- Upnght part, 

 medusan Medusai are included in the sub-class 8cyp]iomedus(B. 



Hydromedusan Medusa. The tiny Meduste given off from the 

 fixed stocks of Hydroid Zoophytes come under two groups. Those 



' From " Proceedings of the Eoyal Society." 

 - Skuphos, a cup ; stoma, mouth. 



