vni. D, 4 Wharton: Some Philippine Thalassemse 259 



with a slight thickening but an almost continuous and uniform 

 layer to a differentiation into distinct bundles may be seen. In 

 T. formosulum a condition midway between the two extremes 

 seems to exist. In the case of T. griffini and of T. erythrogram- 

 mon of Ruppell, if we study only that part of the dermal muscle 

 layer in which the bundles are seen, it would seem to be entirely 

 different from the other forms (figs. 3 and 4), and only a study 

 of the development of the species would show how this differ- 

 entiation has occurred. However, a glance at a section of the 

 body wall taken in the anterior terminal region (fig. 5), in which 

 the longitudinal muscle is continuous and uniform in thickness, 

 will cast grave doubts on the essential difference of this charac- 

 teristic also. From these facts it seems probable that with 

 more material at hand it would be possible to arrange a series 

 showing the gradual differentiation of the muscles from T. 

 neptunii at one end to T. griffini at the other. 



Therefore, it does not seem advisable to take the dermal muscle 

 layer as the essential characteristic for a division of the group. 

 The other character suggested and used by Shipley in his key 

 to the species of Thalassema; that is, the number of pairs of 

 nephridia, seems then to be the most important and essential 

 ontogenetic character on which to base the larger divisions of 

 the group, going back as it does in the ontogeny of the group 

 to the disappearance of the segmentation. It seems probable 

 that the most primitive type in the group is that in which 

 3 or more pairs of nephridia are present; the greater number 

 of pairs, the more primitive the form. Undoubtedly, the sim- 

 plest forms in that group are those which have a continuous 

 and uniform longitudinal muscle layer. From this type the 

 differentiation of the nephridia occurs in two directions. First, 

 that in which more than 2 nephridia are developed in each 

 segment; as, for example, in T. elegans Ikeda. Secondly, that 

 in which the nephridia decrease in number to 2 pairs or 1 pair. 

 As proof of this, a note on the variation of the nephridia in 

 T. neptunii by Stewart (44) is of interest. He found, in dis- 

 secting some specimens of this species from Plymouth, 1 indi- 

 vidual in which a third unpaired nephridium was present, on 

 one side, midway between the first and second pairs. By taking 

 careful measurements on this abnormal individual and a number 

 of other normal specimens, he came to the conclusion that this 

 abnormal nephridium represented a segment, lying between the 

 2 segments which regularly bear the nephridia, from which in 

 normal individuals the original pair of nephridia has been lost. 



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