Structure and Development of the Nephridia. 



399 



Explanation of Plates 22 — 25. 



Ali the figures of microscopie structure were drawn with the aid of the 

 camera lucida under Zeiss objectives; Figa. 35 and 36 with 8 mui. obj., Oc. 6; 

 Fig. 41 with 2 uim. apochromat., Oc. 1 ; Figs. 7, 8 and 29 with 2 nom. apochrom. 

 Oc. 2 ; and the remainder with 2 mm. apochromat. Oc. 4. Finer details were 

 usually drawn in with Oc. 6. 



Eeference Letters. 



The number of the somite is indicated by a Roman numerai. 

 Each nephridium is designated by the letter n with an exponent to indi- 

 cate its number in the larvai stage, thus: n2 = 2^^ larvai nephridium (nephri- 

 dium of somite V) etc. The position of the septum is indicated by the numbers 

 of the somites which it separates, thus: 5 — 6 = Sth septum. 



b.tc Body-wall. 



ce Chloragogen cells. 



c.i.s Circum-intestinal blood-sinus. 



c.ìtì.f Circular muscle-fibres. 



dijdo First and secoiid diaphragms. 



d.h.v Dorsal blood-vessel. 



d.s Dorsal setae. 



d.a.s Dorsal seta-sacs. 



e.d.l Epitheliumof dorsal lip of nephro- 

 stome. 



e.g.x Ectoderm of growìng zone. 



e.v.l Epithelium of ventral lip ofnephro- 

 stome. 



ex.ep Excretory epithelium of nephri- 

 dium. 



ex.g Excretory granules. 



g.e Germ celis. 



glc Ectodermal gland-cells. 



gl. neph Glandulär region of nephri- 

 dium. 



g.^. Growing zone. 



int Intestine. 



i.s Intersegmental septum. 



/ Leueocyte or excretophore. 



l-b.v Lateral blood-vessel. 



l.m.f Longitudinal muscle-fibres. 



fn.c Myoblast cells. 



mn.c Mesenchyme cells. 



m.p.n Mesoderm pole-nucleus. 



m.s Muscular slip attached to dorsal 

 lip of nephrostome. 



mfh Mouth. 



m.t.v Muscle-fibres of terminal vesicle. 



m.v.l Membranous wall of vesicle of 

 ventral lip of nephrostome. ' 



n.b.v Nephridial blood-vessel. 



nst.ep Epithelium of nephrostome. 



nst.v Nephrostomial blood-vessel. 



oes Oesophagus. 



pb Proboscis. 



pd Proctodaeum. 



p.d.l Ciliated processes of dorsal lip 

 of nephrostome. 



p.m Post-mesodermal region of poste- 

 rior extremity. 



jm Peritoneum. 



pn. f Peritoneal fold forming nephridial 

 blood-vessel. 



prosi Prostomium. 



s Septum. 



sh Seta-forming ectodermal celi ('seti- 

 blast'). 



s.b.v Segmentai blood-vessel. 



s.oe.g Supra-oesophageal ganglion. 



st Stomach. 



t.m.f Transverse muscle-fibres. 



t.v Terminal vesicle. 



v.b.v Ventral or sub-intestinal blood- 

 vessel. 



t.b.w Ventral body-wall. 



T.s Ventral seta. 



v.s.s Ventral seta-sac. 



v.v.l Vesicle of ventral lip of nephro- 

 stome. 



