218 PHYLUM ANNELIDA. 
posterior surfaces of the septa between segments 9-10 and 
10-11. They are minute, translucent, and difficult to see. 
In immature worms they lie exposed in the body cavity; in 
mature worms they are concealed by the great development 
of the seminal vesicles. 
(2) The seminal vesicles are much-lobed structures, 
exceedingly prominent in dissection. Small and laterally 
placed in young worms, in the adult the anterior two 
Fic. 113.—Reproductive organs of earthworm.— 
After Hering. 
N., Nerve cord; 7., anterior testes; S., sacs of sete; 2.S., 
receptacula seminis; 5,4, seminal funnels; v.d., vas deferens ; 
ovd., oviduct; ov., ovary; sv, seminal’ vesicles cut open; 
Vill. -XV., segments. 
pairs fuse in the middle line and cover the anterior pair 
of testes and its ducts, while the posterior pair similarly 
conceals the second pair of testes with its ducts. Into the 
seminal vesicles mother sperm cells from the testes pass, 
and there divide up to form spermatozoa. 
Development shows that the seminal vesicles arise as 
outgrowths of the septa of segments 9-12, and that their 
lumen is a portion of the body cavity. This is of importance, 
