THE ANCESTRY OF THE CHORDATA. 543 
worms!. Thus, in animals whose long axis has been sup- 
pressed, it appears that repetition may arise of most of the 
organs of the body radially arranged; next, that not only the 
specific but also the individual number of these variations is 
liable to great variations, pointing to the fact that the power 
to repeat in this way is one which may be easily called into 
action producing great differences of form. 
It may also be observed in this connection that similar 
casual repetitions are frequent in the case of the Gonozooids 
of Hydromedusa, in which animals also they are radially 
arranged. As in the case of the Echinodermata this is shown 
by the great diversity in the specific and individual number of 
those organs which are radially repeated. The latter may be 
seen, for example, in Clavatella prolifera. The Medusa of 
this animal creeps about on its tentacles, which are long and 
stiff, and which carry short suctorial processes on their oral 
faces which support the animal, giving it the appearance of an 
Ophiurid. The number of these tentacles and of the radial 
canals varies with age, from six to eight (Hincks). In the 
specimens which have come under my own observation in the 
undetached buds the number of these arms was five, while 
those of the free Medusa was generally six. The number of 
the organs in Cladonema radiatum, another creeping form, 
is also very variable, the number of oral lobes being five or 
seven, and that of the tentacles and canals eight or ten 
(Hincks). 
The facts of Echinoderm and Ceelenterate anatomy above 
quoted, suffice to illustrate the statement that in animals 
whose organs are already radially repeated, variations con- 
sisting in the repetition of one or more of the peripheral 
organs is of common occurrence, and may affect large numbers 
1 Tn relation to this acquisition of the appearance of longitudinal repetition 
or segmentation by a radiate animal, an example of the inverse phenomenon 
may be given. Among the Operculate Cirripedes, though in the Balanide the 
arrangement of the six plates composing the “cone” are so placed as plainly 
to indicate the original long axis, yet in the Coronulide this feature becomes 
obliterated, and the plates are disposed in a radially symmetrical manner. 
