TYPES OF C@&LENTERA—A MEDUSOID. 151 
tentacles. The mesogloea forms a thickened jelly, present 
more especially on the convex (ex-umbrellar) surface. 
The mouth opens into the canal of the manubrium, which 
leads to the central cavity of the dome. With this a varying 
number of unbranched radial canals communicate; these 
open into a marginal circular vessel, which communicates 
with the cavities of the tentacles. A plate of endoderm lies 
in the mesogloea between the radial canals. Digestion is 
intracellular, and probabiy goes 
on throughout the whole of this 
‘gastro-vascular” system. 
The movements of the bell 
are caused by the contractions 
of the ectodermic muscle cells. 
The nervous system consists 
of a double ring of nerve fibres 
around the margin of the bell. 
With these are associated gang- 
lionic cells, which apparently 
control the muscular contrac- 
tions. Sete rr 
IG. 72.— structure of a 
a miter bet ot ey eee M alse —After ee 
base of the tentacles (Ocellatz), Ae T ernen ee. coded 
or in the form of “auditory” ference canal; G., gonad; &.C., 
. ix ahe radial canal; 4WV., endoderm; 
vesicles developed as pits in the | ZC., ectoderm; 4/G’, mesoglea. 
velum (Vesiculatz). 
The reproductive organs develop either in the manu- 
brium or on the radial canals. The products always (?) 
ripen in the ectoderm, and often seem to arise there; but 
Weismann and others have shown that the reproductive 
cells of a medusoid derived from a hydroid, or of the 
reduced and fixed reproductive persons of many hydroids, 
have considerable powers of migration, and may originate 
(sometimes in the endoderm) in the hydroid colony at 
some distance from the place where they are matured within 
the medusoid bud. The sexes are usually separate. The 
commonest kind of free-swimming larva is the planula, which 
is oval, ciliated, and diploblastic, devoid of an opening, and 
usually without a central cavity. In the case of those 
medusoids which arise as liberated sexual members of 
