224 



INVERTEBRATE MORPHOLOGY. 



organ's both lying in the anterior portion of the body, usually 

 between the ninth and fourteenth metameres, or sometimes 



even farther forward. There are 

 either one or two pairs of testes 

 (Fig. 104, t), which arise like the 

 ovaries from the peritoneal epithe- 

 lium and early break down to a 

 greater or less extent, their cells 

 passing into seminal vesicles {vs), 

 where they undergo further de- 

 velopment into spermatozoa. The 

 vasa deferentia {yd) are modified 

 nephridia, and are either two or 

 four in number according as there 

 are one or two pairs of testes. 

 "When four are present they may 

 open separately, or may unite in 

 pairs on either side in a common 

 atrium, through which they open 



to the exterior, or finally those of 



Fig. 104. — Nervous System ,, . , ./ , , 



AND Repkoductive ORGANS ^^^ Same side may unite a short 

 OP Lumtn-icus. distance below the funnels, forming 



ce = supraoesophageal ganglion, for the greater part of their course 



n = ventral nerve-cord. 



od = oviduct. 



ov = ovary. 



ra = receptaculum seminis. 



so = subcEsophageal ganglion 

 t = testis. 



vd = vas deferens. 



m = vesicula seminalis. 



a single tube. There is only a 

 single pair of ovaries (ov), to which 

 in some forms ovarian receptacles 

 similar to the seminal vesicles are 

 added ; and in all but some of the 

 lower forms oviducts (od), which 

 are modified nephridia, are pres- 

 ent. In front of the metameres which bear the testes one, 

 two, or occasionally three pairs of invaginations of the body- 

 wall occur, producing pouches projecting into the bodj-- cavity 

 — the seminal receptacles (rs)— which receive the seminal 

 fluid during the mutual interchange of it which takes place on 

 copulation. 



A satisfactory subdivision of the Oligochaeta into orders has not yet 

 been possible ; indeed the various families are so related to one another 

 that such a subdivision seems unnecessary. Formerly it was the custom 



