ANNELIDA 327 



formed from the three macromeres have been added others, which hke- 

 wise have probably been furnished from the same source. The entoderm 

 now hes in the form of two rows of cells upon the macromeres (Fig. 154 

 A), which now represent a kind of food-yolk. They are surrounded and 

 partly covered in by the germ bands, while the ectoderm, now increasing 

 more rapidly, covers over the anterior part of the embryo. A central 

 fissure (the fundament of the cavity of the mesenteron) soon arises be- 

 tween the entoderm cells, which enlarge at the expense of the yolk-cells 

 [macromeres] (Fig. 154 A). The latter are forced more toward the hind 

 end of the embryo, and are finally overgrown by the ectoderm, which 

 also spreads out backwards (Figs. 154 B, 156). In this case, there- 

 fore, the macromeres are not taken into the intestine, as in Clepsine, 

 but remain outside of it ; but in this position they too are gradually 

 absorbed. The mouth and pharynx finally arise at the anterior end of 

 the embryo in the form of an ectodermal invagination, which unites with 

 the intestine (Fig. 154 B). 



2. The Larvae of the Gnathobdellidse. 



The embryos of the Gnathobdellida3 break through the 

 egg- membrane at a stage iu w^hich they are spherical or oval 

 and have attained about the condition represented in Fig. 

 154 B. The pharynx, still very simple in structure, leads 

 into the intestine, which now begins to enlarge. The ecto- 

 derm has not yet quite grown over the macromeres. The 

 " germ bands " lie between it and the entoderm. It is seen 

 that the development is not so far advanced as that of 

 the hatching embryo of Clepsine. Whereas the latter is 

 converted directly into the worm, the embryo of the 

 Gnathobdellida? undergoes a protracted lai'val existence. 

 Like the larvae of the Oligochaeta, those of the Gnathobdel- 

 lidse float in the albumen of the cocoon, and take this into 

 the intestine by means of deglutitory movements. For this 

 purpose a provisional phai-ynx (Figs. 154 B and 156 s) is 

 developed, which is provided with a powerful musculature. 

 The larva possesses still other provisional structures which 

 are entirely wanting in Clepsine. In Nephelis a cephalic 

 process is developed, which is thickly covered with cilia (Fig. 

 156). This ciliation recalls that which occurs in the larvae 

 of the OligochiBta, especially since, as in Lumhricus trape- 

 zoides, it extends on to the ventral side, where it is found in 



