ANNELIDA 337 



sacker — which cause it to resemble the Hii'üdinea (Voigt, 

 Vejdovsky). The development exhibits in some features a 

 resemblance to that of the Hirudinea, but otherwise it 

 is so peculiar — provided we can rely on the statement of 

 Salensky — that the relationship of Branchiobdella to either 

 branch of the Annelida cannot be inferred from it. 



Branchiobdella lays its eggs, each surrounded by a firm 

 envelope, on the gills of the ci'ayfish, where they are 

 attached by means of a stalk, a prolongation of the 

 envelope. A cocoon proper, as in the OligocliEeta and 

 Hirudinea, does not exist, although the egg is surrounded, 

 as in these, by a special envelope ; perhaps therefore tlte 

 outer envelope is equivalent to a cocoon. 



In the cleavage and the formation of the germ-layers, 

 conditions are exhibited which do not resemble those of the 

 Hirudinea or Oligochaeta, but can perhaps be referred moi'e 

 readily to the latter. We begin with the stage in which one 

 large and three small blastomeres are formed. All four are 

 to be called macromeres, for soon four inicromeres are 

 abstricted from them. By division of the micromeres and 

 the formation of new ones on the part of the macromeres, a 

 rapid increase of these small (ectoderm) cells takes place. 

 They soon form an irregularly defined cell-plate, the sides of 

 which grow out and overlie the macromeres in the form of a 

 cap. The striking thing in this is that the micromeres are 

 said to correspond to the ventral side of the Worm ; however, 

 it is also stated that for Clepsine the mouth breaks through 

 in the region of the first four micromeres, and it has a 

 similar position in Nephelis (comp. Fig. 154, p. 326). A 

 rather small cleavage cavity makes its appearance between 

 the micromei'es and macromeres; it is subsequently forced 

 away from the macromeres by the production of new 

 cells. The macromeres have likewise divided and arranged 

 themselves as two pairs of large cells at the posterior end 

 (Fig. 157 A). A cord of ectoderm cells forces its way 

 between the two pairs. The double-row arrangement of the 

 macromeres persists even when they divide further (Fig. 

 157 B). 



K. H. B. Z 



