THE ANCESTRY OP THE OHORDATA. 73 



plicated organs as shells may be repeated within the limits of 

 a small group. Moreover, in some Chitons bunches of cal- 

 careous setse 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 ^olidse, 

 being in some (as ^olis papillosuSj ^olis pulcher, &c.) 

 arranged in more or less regularly paired oblique rows, while 

 in others (as Dendronotus) the liver caeca 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 of 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 



