ANNELIDA 



297 



becomes covered with peritoneum (Fig. 137 (7, pt), and 

 presses its way out towards the ectoderm, in order to fuse 

 with it directly, or with an invagination of it (Bergh, 

 No. 7), which forms the terminal portion of the duct, or 

 collective vesicle, when such is present (Yejdovsky, No. 43). 



According to the observations of E. Meyer (No. 31), the thoracic 

 nephridia of Psygmobranchus are composed of separate parts. The 

 nephridial ducts arise from large mesoderm cells, which are found in the 

 blastocoele of the larva ; the 

 funnels, on the contrary, and 

 the peritoneal covering of the 

 nephridia are supplied later by 

 the primitive segments. The 

 ends of the nephridial tubes 

 open to the exterior by means 

 of provisional pores, which 

 later occupy the floor of a 

 ciliated groove, which closes, 

 and represents the unpaired 

 ectodermal efferent duct of 

 these two so peculiarly con- 

 stituted nephridia of Psygmo- 

 branchus. 



Genital Organs. — The 

 development of the sexual 

 glands is very simple in 

 both the Polychceta and 

 Oligochceta. They arise as 

 growths of the peritoneal 

 epithelium on the septa, 

 or, as frequently in the 

 Polychceta, on the invest- 

 ment of the blood-vessels. 

 The genital gland, which 

 in Lumhricus is distin- 

 guishable even during 

 cocoon life (Bergh), sepa- 

 rates from the peritoneum as the result of a rapid prolifera- 

 tion of the cells, and gradually assumes its permanent form 

 (Fig. 138 A to jD, after E. Meyer). The genital products are 



Fig. 137.—^ to C, parts of longitudinal 

 sections through embryos of Crwdrilus, 

 showing the developmetit of the nephridia 

 (after Bebgh). ed, ectoderm ; 7i, cavity of 

 primitive segment ; pt, peritoneum (of 

 the nephridia) ; s, septa ; so, somatic, sp, 

 splanchnic layer of the mesoderm; t, 

 funnel ; tz, funnel cell. 



