276 



THE INVERTEBRATA 



the tube consists of ectoderm : the funnel of the nephrostome in Lum- 

 bricus (and probably of other forms) is derived from a single ecto- 

 dermal cell. The coelomoduct is entirely formed from mesoderm and 

 usually has a wide coelomic funnel easily distinguished from the 

 typical nephrostome. The oviducts and the sperm ducts oi Lumbricus 

 are coelomoducts. In a family of the Polychaeta called the Capitel- 

 lidae there are coelomoducts in most segments of the body serving as 

 gonoducts (Fig. 191 I, D). In the majority of Polychaeta they have 



co.d- 



nep. 



-nmx._ 



cor. 



,nephr. 



co.d. 



Fig. 191. Segmental organs of Polychaeta. After Goodrich. I, Transverse 

 section (right half) of body segment showing combinations of nephridia and 

 coelomoducts. A, Hypothetical. B, Phyllodocidaeand Alciopidae. C, Neph- 

 thyidae and Glyceridae. D, Capitellidae. E, Capitellidae. F, Nereidae. 

 co.d. coelomoduct; cor. ciliary organ; nep. "closed" nephridium; nep.o. 

 "open" nephridium; nmx. nephromixium. II, Segmental organ of Vanadis 

 (Alciopidae). nephr. coelomic funnel; sol. solenocytes. 



however disappeared altogether and their function is otherwise 

 performed. 



There is in addition a type of organ called a nephromixium which is 

 formed by the union of a nephridium and coelomoduct. In the 

 Alciopidae the separate components of the nephromixium are clearly 

 seen (Fig. 191 II). Here the union is with the closed nephridium but 

 in the Capitellidae and many other polychaet families there are 

 open nephridia and these have often an intimate fusion with the 

 coelomoducts. Thus in one form of the Capitellidae shown in Fig. 



