The Development of Ischnochiton. 645 



Commencing with the Annelid at the stage represented by- 

 diagram E, a, the first quartette also forms the head vesicle and the 

 second and third quartettes occupy the same relative positions as in 

 the Chiton larva. In Lepidonotus and Podarke the second quartette 

 cells are of the same size at the time of their formation but in 

 Amphitrite for example a modification has arisen whereby 2d (first 

 somatoblast) is much larger than its fellows. Correlated with its large 

 size the cell exhibits a rapidity of development whereby its progeny, 

 before the mouth has reached its permanent position, compose more 

 than half of the lower hemisphere of the embryo (E, b). 



Nevertheless if we compare Amphitrite at this stage with E, c 

 of Ischnochiton, it will be seen that essentially the same condition of 

 affairs exists. In each the embryo is at first radial; the three 

 quartettes of ectomeres in each not only occupy the same relative 

 positions but as has been previously shown the cleavages during this 

 period are in very many cases identical. The mouth shifts in the 

 same fashion in each and during its migration the various quartettes 

 behave in essentially the same manner. 



Precocious segregation has brought about modifications of the 

 first somatoblast in both forms, in the Annelid to the greatest degree, 

 yet the origin, development and relations of these groups show them 

 to be wrought upon the same plan. Indeed in the face of the long 

 continued resemblances which have been shown to exist between the 

 embryos of the Chiton and Annelid it is impossible not to believe that 

 they are descendants of a common ancestor. And it appears very 

 probable that the relationship does not stop here but extends to the 

 Gastropods and Laraellibranchs and probably includes the flatworms. 

 The later stages in these forms are not so thoroughly investigated 

 yet they give every indication of following along the line of develop- 

 ment taken by the Annelid and Chiton. 



6. Metamerism. 

 It is not the function of this paper to discuss even the most 

 important views regarding the significance of the trochophore, the 

 origin of metameric segmentation, and related problems. These have 

 been presented in an admirable fashion by Fratpont ('87) and 

 Hatschek ('88) and some points have been added by Whitman ('87) 

 and Wilson ('89). It is intended simply to present a few consider- 

 ations relating to Ischnochiton development which in their relation to 

 the embryology of Annelids appear of significance. I fully appreciate 



