5o6 



THE AMERICAN NATURALIST. [Vol. XXXVI II. 



larvae, is undoubtedly of ectodermal origin in Fiona. It arises 

 from one large and several small third quartet cells of the right 

 posterior quadrant which, after torsion begins, are carried far- 

 ther to the right side and above the anal opening. The larva 

 also possesses additional organs of excretion, two in number, 

 lying bilaterally in the anterior region of the body cavity. 

 These, the " nephrocysts," are composed of but a few large 

 vacuolated cells which contain concretions and fluid excreta. 

 The origin of these cells has not been determined definitely, but 

 they are probably derived from the mesoderm. 



A Pair of Gic 



'cessory Chromosome in the Spider: 

 sity of Pennsylvania. — In the sper- 

 lumber of authors have described a 



peculiar chromosome which has been called by various names. 

 While these all agree in being unlike the ordinary chromosome, 

 there seems to be sufficient disagreement to subdivide them into 

 two groups. Montgomery's " chromatin nucleolus "^and Paul- 

 mier s " small chromosome " are probably homologous and these 

 authors have suggested that they may represent degenerating 

 chromatin. McClung's " accessory chromosome " in Orthoptera 



