336 EMBRYOLOGY 



General Considerations. — The development of the 

 Hirudinea doabtless points to the fact that in dealing with 

 them one has to do with Annelida. Although differing in 

 details, the entire process of development is similar to that 

 of the Chaetopoda, and especially of the Oligochaeta. The 

 so-called germ bands of the Hirndinea and the mesodermal 

 bands of the Chaetopoda, it is true, do not appear to be 

 homologous structures, but the entire manner of their 

 formation and their relation to the embryonic body in 

 general, as well as their subsequent development, indicate 

 that both are to be referred to like structures, and that in 

 the Hirudinea a modification has appeared only in so far 

 as the more simple mesodermal bands have there acquired a 

 more complicated structure by the addition of ectodermal 

 parts. In their mode of development the Hirudinea appear 

 to be less primitive forms than the Chaetopoda. 



Just as the mode of origin of the individual organs, 

 especially the body cavity, the nervous system, and the 

 excretory system, shows the leech to be an Annelid, so, too, 

 does its anatomical structure. This is mentioned only for 

 the reason that direct relationships between the Hirudinea 

 and Platyhelminthes have been sought for in various direc- 

 tions. In this connection it is only the structure of the 

 genital organs and their resemblance to those of the 

 dendrocoelous Turbellarians that appear to be remarkable. 

 It would be desirable to know more than we do at present 

 regarding this point. 



In brief it must be said that, as compared with the 

 Chaetopoda, the Hirudinea show themselves to be in struc- 

 ture and development higher forms, which exhibit many 

 secondary modifications. 



VI. BRANCHIOBDELLA. 



The systematic position of Branchiohdella is not yet 

 established. There are anatomical grounds for the view 

 that this worm is to be assigned to the Oligochaeta, and that 

 it is only in consequence of its parasitic mode of life that it 

 has acquired certain characters — for example, the posterior 



