ANNELIDA 



333 



328), finds a cex'tain resemblance between the primitive 

 kidneys and the permanent nephridia, in that both of them 

 arise £i*om the same parts, i.e. from the middle rows of the 

 g-erm bands. However, in the Rhyncobdellidce themselves, 

 which Whitman studied, primitive kidneys are not present. 

 The origin of the nephridia from a continuous cord of cells, 

 which, moreover, is described by Wilson in the same way 

 for Lumbricus, recalls the theory advanced by Hatschek that 

 in Criodrilus the permanent nephridia arise from a cord of 

 cells in the somatic layer of the mesoderm (comp, sttpra, p. 

 296). 



The development of the nephridia takes place from in 

 front backwards by the cord of cells becoming many- 

 layered and undergoing a segmental division. How the 

 nephridia arise from the cell-masses thus produced has not 

 yet been accurately determined. A pair of nephridia is 

 begun in each segment, though all of them do not develop, 

 for in the adult worm there are only sixteen pairs. ^ 



The Body-cavity and its Lining ; Mnsctdatiire ; Blood-vessels. 

 — The peritoneal lining of the body cavity and the somatic 

 and intestinal musculature arise from the two more deeply 

 located cell-rows of the germ band, the pole cells of which 

 we have learned to designate as the mesoblasts. The two 

 cell-rows have changed into voluminous cords of cells by the 

 rapid multiplication of their elements. These cords undergo 

 a segmentation from in front backwards. The primitive 

 segments thus produced extend out dorsally, and cavities 

 make their appearance in them. The latter correspond to 

 the segmental (metameric) cavities of the Cluetopoda. 

 After growling completely around the intestine, they are 

 said to become confluent, and to form the marginal sinus, 

 which belongs to the lacunar portion of the blood-vascular 



1 [The formation and differentiation of the rows of cells produced by 

 the teloblasts has been again traced by Bergh and by Apathy, as well as 

 in the works of Whitjiax (see Appendix to Literature on Annelida). The 

 subjects involved are the formation of the nervous system, the body 

 musculature, and the nephridia. These organs have been traced back to 

 definite parts of the so-called germ band, though as yet complete agree- 

 ment on the part of the authors has not been reached. — K.] 



