PHYSIOLOGY OF THE INVERTEBRATA. 425 



(yj- in. long), situated in the thirteenth segment. The ova 

 when fully developed rupture the walls of the ovaries, and 

 pass into the body cavity. Ultimately the ova find their 

 way into the oviducts, which are quite distinct from the 

 ovaries. The pair of oviducts are small, fannel-shaped, 

 ciliated tubes; the funnel-shaped portion opens internally 

 in the thirteenth segment, whereas the opposite end opens 

 externally by a small aperture on the ventral side of the 

 fourteenth segment. 



Although Lumhricus is hermaphrodite, copulation takes 

 place between two separate individuals, the impregnating 

 seminal fluid being stored in four small spherical sacs or 

 spermathecEe, which open externally between the ninth and 

 tenth, and the tenth and eleventh segments. The ova are 

 impregnated externally by the seminal fluid from the sperma- 

 thecas. Groups of these ova become surrounded by mucous 

 and chitinous secretions termed cocoons. These cocoons 

 sometimes contain forty or fifty ova, an albuminous substance, 

 and packets of spermatozoa. In the development of the 

 Oligochceta, the segmentation of the ovum is total and nearly 

 regular, giving rise to a flattened ciliated blastosphere. The 

 latter invaginates, and a gastrula is formed; and between 

 the epiblast and hypoblast in this stage of the development a 

 mesoblast is formed. " On the ventral side of the embryo, 

 the mesoblast divides into a row of quadrate masses, which 

 are symmetrically arranged on each side of what becomes the 

 median line of the adult body. This series of symmetri- 

 cally placed quadrate masses resembles the protovertebree of 

 the embryo of a Vertebrate animal." After this " a cavity 

 forms in the interior of each quadrate mesoblastic mass, 

 making it into a kind of sac. The adjacent anterior and 

 posterior walls of the row of sacs unite, and this union gives 

 rise to the dissepiments of the somites, while the cavities 

 become the body cavity or perivisceral chambers." (Johnstone.) 

 The epiblast now thickens inwardly, along the median 

 line, ultimately, giving rise to the ganglionic nervous 



