324 EMBRYOLOGY 



also to undergo its further development while attached to 

 the ventral surface of the parent. As regards the external 

 form of the body, a segmentation [metamerism] can be recog- 

 nized, which is to be referred to that of the germ bands 

 (Fig. 152 F). This segmentation makes its appearance in 

 the same way as in the Chaetopoda, namely, progressing 

 from in front backwards.^ Furthermore the shape changes, 

 in that the body, which was first flat on the dorsal and 



strongly curved on the ventral 

 , s. d.- 0-- *■-• surface (Figs. 152 i^ and 153), 



becomes straight and flat on 



the ventral surface, while its 



\ ^^^^^^Hl^/ dorsal side assumes the fami- 



l^^f^ liar arching owing to the more 



active growth of that part. 



Fig. 153.-Kmhryo of clepsine (after At this stagO the body COnsists 



RATHKKandWHiTiiAM). d, intestine; of thirty-three Segments, the 



fc."*, g-erm bands; s, pharynx; sn, , . • •, , p i • i -i 



gupjjgj. posterior eight ot which unite 



to form the posterior sucker 

 (Fig. 153). The anus arises dorsad of this by the fusion of 

 the entoderm and ectoderm. 



It is a question how the formation of the germ-layers is to be ex- 

 plained. Whitman assigns to the ectoderm the cells that have arisen by 



^ [Whitman (No. XXII., Appendix to Literature on Annelida) has 

 recently investigated the metamerism of the Hirudinea and its origin. 

 He endeavors to explain the segmentation of the adult animal by 

 means of embryological facts, and further supports his opinion by the 

 anatomical conditions, especially that of the nervous system. The 

 principal question concerns the interpretation of the head, which is 

 composed of the primary head segment and several trunk segments 

 united with it, a condition similar to that which is also assumed for 

 the Chaetopoda. The mouth may be placed as far back as the fourth 

 segment. 



The segmented body is derived from an unsegmented. The origin of 

 the segmentation is to be sought in the reproduction by division of the 

 originally unsegmented worm. The individual segments therefore really 

 correspond to separate individuals. The increase in the number of 

 segments is caused by the method of life, which necessitates such an 

 increase. (Comp. in this connection the statements made under " General 

 Considerations regarding Annelida," p. 348.) — K.] 



