72 WILLIAM BATBSON. 



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 repetitious 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 

 '' segmented;" 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 tliis 

 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- 



