540 WILLIAM BATESON. 
extremely common, and appear to arise suddenly and in forms 
nearly allied to those in which they are not found. 
Firstly, among the ciliated Platyhelminths a striking case is 
offered by Gunda segmentata, in which, as described by 
Lang, the diverticula of the gut, the testes, the yolk-glands, 
the tubules of the excretory organs, the transverse commis- 
sures, and the nerve-cord, are all regularly and synchronously 
repeated. Now, this case stands alone merely in the com- 
pleteness of the repetition. All through the Turbellaria are 
to be found many instances of animals with great numbers of 
gut diverticula, with testes and yolk-glands scattered all over 
the body, with branched excretory systems, with anastomosing 
nervous networks, &c. Not only this, but instances are 
common in which some of these structures are repeated regu- 
larly, and others irregularly or not at all, as, for example, 
Polycelis pallida (Quatrefages), in which the ovaries are 
scattered and the testes are not, while the reversed condition 
is more frequent. It becomes probable that the repetitions of 
these organs did not phylogenetically occur simultaneously, 
_ but that repetition occurred at various times in each set of 
organs. 
Again, among Nemertines in some species the saccules of 
the gut, the generative organs, and the circular blood-vessels 
are all repeated together and with great regularity, so as to 
produce a segmented whole. In other species these repetitions 
are not all formed or are more or less irregular, thus pointing 
to the fact that these repetitions have been acquired within 
the limits of the group. The development (v. especially 
Salensky, ‘Arch. de Biologie,’ 1884) precludes at once the 
possibility of the ancestral form of Nemertines having been 
‘“ seomented ;”’ hence they, together with the Planarians, offer 
a type of a high degree of repetition being acquired within the 
limits of a group. Nor do these forms alone exhibit this 
feature as one peculiar to themselves, for there are few groups 
in which it is not found. Even among Mollusca, which are, 
perhaps, the most typically unsegmented of all forms, the 
Chitons may be instanced as examples showing that such com- 
