HYDROZOA. 
55 
from the parent colony and swam away. In the case of many Case 3, 
Medusae, the eggs develop directly into Medusae, no fixed stage ^ & ^ 
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. Medusae 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 Medusae), and the fixed stock, 
when known, is of the Hydra or Hydroid type, and produces buds 
laterally. Medusae of this type come under the sub- class Hydro- 
medusco, which includes Hydroida and Hydro coralline. 
In Medusae of the second or “ Scyphomedusan ” type the 
Fig. 17. ] 
Female Medusa of Millepora, showing the eggs and marginal batteries 
of thread cells. (After Prof. S. J. Hickson.) 
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 1 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- u pright P art - 
medusan Medusae are included in the sub-class Scyjohomedusm. 
Hydromedusan Medusa. The tiny Medusae given off from the 
fixed stocks of Hydroid Zoophytes come under two groups. Those 
1 From “ Proceedings of the Royal Society.” 
2 Skuphos, a cup ; stoma , mouth. 
