THE ANATOMY. REPRODUCTIVE SYSTEM 87 



thence either directly to the exterior or into the mouth of the egg-sac which lies 

 conveniently near to the funnel. Among the Eudrilidae the passage of ova is 

 enormously facilitated by the fact that the ovaries are enclosed in special peritoneal 

 sacs which are continuous with the efferent duct ; these sacs are late in their 

 development, which seems to indicate their comparatively modern appearance; they 

 were first discovered by myself in the genus Eudrilus (62); since that time they 

 have been recognized in the majority of the Eudrflidae (which see for a more detailed 

 description). 



Ova. — The parasitic and encysted Gregarines were at one time mistaken for the 

 ova of Lumhricus — perhaps not altogether an unnatural error. The ova are now 

 known in a large number of Oligochaeta; but it is in Rhynchelimis that they have 

 been most thoroughly studied (by Vejdovsky). It is a remarkable fact that the 

 Oligochaeta can be divided into two groups according to the character of their ova; 

 in the aquatic Oligochaeta the eggs are large and contain an abundance of yolk ; in 

 the terrestrial forms, on the other hand, the ova are of microscopic size and contain 

 but little yolk. It is, it will be observed, the large forms which have small eggs 

 and broadly speaking the small forms which have large ova. All the genera which 

 were grouped by Clapaeedb within his group 'Limicolae' have large ova containing 

 much yolk. Differences of size no doubt occur in both groups ; but in no earth- 

 worm is the egg ever so large as it is in the aquatic worm with the smallest ova. 

 The difference is a remarkable one ; it is almost, if not quite, as striking as that 

 between the ova of a Mammal (not of course Prototherian) and a Frog. The large 

 size of the ova was justly made use of by d'Udekem in his classification of the 

 Oligochaeta. 



It is difficult to account for this striking difference; Vejdovsky (24) pointed out 

 that there is a difference in the mode of nutrition in the ova in the two groups ; in 

 the aquatic Oligochaeta the eggs are early detached from the ovary and undergo 

 further development in the body cavity or in egg-sacs ; on the other hand, in 

 earthworms the eggs reach maturity in the ovary which is furnished with abundant 

 blood-vessels, which are wanting in the egg-sacs of the others ; this latter statement 

 is not quite accurate — at least in one way; it is perfectly true that there is 

 no development of blood-capillaries on the egg-sacs of those worms ; but special 

 perivisceral vessels undergo an increase of length at the time that the egg-sacs are 

 formed and accompany them. This cannot, however, be regarded as an explanation 

 now; for in Eudrilus, and in the Eudrilidae generally, the eggs are apparently 

 transferred at an early stage to the egg-sacs where they reach maturity; indeed, in 

 some Eudrilidae the ovaries are even at an immature stage no longer to be found 



