ANNELIDA 337 



sucker — which cause it to resemble the Hirudinea (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 crayfish, where they are 

 attached by means of a stalk, a prolongation of the 

 envelope. A cocoon proper, as in the Oligochaeta and 

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

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

 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 more 



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



large and three small blastomeres are formed. All four are 



bo be called macromeres, for soon four micromeres are 



ibstricted from them. By division of the micromeres and 



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



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



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



'hich grow out and overlie the macromeres in the form of a 



iap. 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 Clepsme the mouth breaks through 



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



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



ither small cleavage cavity makes its appearance between 



bhe micromeres and macromeres; it is subsequently forced 



tway from the macromeres by the production of new 



^ells. 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 



lacromeres persists even vrhen they divide further (Fig. 



L67 B). 



K. H. E. Z 



