THE ANCESTRY OF THE CHORDATA. 541 
plicated organs as shells may be repeated within the limits of 
a small group. Moreover, in some Chitons bunches of cal- 
careous sete recur along the sides symmetrically to the 
scutes, producing an appearance not far removed from that of 
Arthropoda. 
Another case is to be found among the Nudibranchs, in which 
the liver diverticula, which are peculiar to and characteristic of 
the group, not only recur in an obviously segmental manner, 
but may be arranged in several ways among the Molide, 
being in some (as Alolis papillosus, Molis pulcher, &c.) 
arranged in more or less regularly paired oblique rows, while 
in others (as Dendronotus) the liver ceca stand in paired, 
arborescent tufts, which are as definitely symmetrical in their 
repetition as any system of organs of a Vertebrate. In cases 
of this kind the regularity of these repetitions is obviously 
secondary, and all the other anatomical features show no trace 
of segmentation, which constitutes the great interest of cases 
of this kind from the point of view of the present argument. 
The cases which have been so far mentioned have all been 
selected from bilateral animals, with a definite long axis in 
the direction of which they move. But the belief that repe- 
titions of this sort are of constant occurrence as a factor in 
effecting modifications of general form, derives most remark- 
able support from the facts of the anatomy of radiate animals, 
especially cf the Echinodermata. From embryonic evidence it 
may be regarded as almost certain that these animals are 
descended from a bilateral ancestor, and that their present 
form has been since acquired. Whenever this change took 
place it came to pass in some entirely unknown manner that 
the various organs came to be repeated round a central 
axis. However this may have been brought about, the fact 
remains that the number of such repetitions did not become a 
fixed and definite feature common to all the divisions of the 
group. For while the number five appears to be the limit of 
the repetition in the Echinoidea, Ophiuridea, and Crinoidea, 
among the Asteroidea the arms of different genera have not 
the same number, nor do they necessarily occur in multiples 
VOL, XXVI, PART 4,—NEW SER. NN 
