COELENTERATA 55 
the hydranth, nor are there capsules surrounding the gono- 
phores. 
The medusae may be well developed and free, or they may be 
permanently attached to the hydranth (gonophore), or they may 
not develope beyond rudimentary 
buds (sporosacs), and finally, 
in Hydra they are not formed 
at all. When free, they may 
be provided with ocelli at the 
base of their marginal tentacles, 
usually on their outer surface. 
The sexual cells are arranged 
round the manubrium, either 
uniformly or in bands. Most 
Anthomedusae are small. 
The tentacles of the hy- Tee LOr 
droid are nearly always solid, IV.—Sub-umbrella cavity opened, 
Hydra, however, is an exception erp mete Ceara ca 
I g 
to this rule; they are usually Ectoderm. 
arranged in one circle as in Hydra, 
Endoderm. 
Structureless lamella. 
or in two as in TZubularia; in Manubrium. 
: 6 Radial canal. 
the family Clavidae, however, Vole 
Gelatinous tissue. 
they are irregularly scattered over 
Sub-umbrella cavity. 
the surface of the hydranth. 
There is no special aggregation of nerve cells in the 
hydroid stage, but in the medusa a nerve ring is present round 
the edge of the umbrella; it is split into an upper and lower 
nerve ring by the insertion of the velum. ‘The ocelli consist 
of a collection of pigment spots, and a cuticular lens is present 
in Jizzia. The sexes as a rule are separate, the genus Hydra 
again forming an exception. Reproduction by fission is rare, 
but sexual reproduction by budding is common in both the 
hydroid and the medusoid stage. When it takes place in the 
latter, a medusa is invariably the result. 
ee ENTED CNG fo) ed 
Order 2. Calyptoblastea-Leptomedusae. 
CHARACTERISTICS.— The hydroid forms of this genus never have 
more than one row of tentacles round the mouth. The 
