THE ANCESTRY OP THE CHORDATA. 75 



worms^. Thus, in animals wliose 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 Hydromedusse, 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 (Hiucks). 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 vei'y 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 Coelenterate 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 In 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 Balanidae the 

 arrangement of the six plates composing the " cone" are so placed as plainly 

 to indicate the original long axis, yet in the Coronulidee this feature becomes 

 obliterated, and the plates are disposed in a radially symmetrical manner. 



