THE ANATOMY. REPRODUCTIVE SYSTEM 95 



Naidornorpha the egg-sac is a single structure surrounded anteriorly by the sperm- 

 sac ; the same statement holds good for other Oligochaeta, indeed for the majority 

 of the aquatic forms ; in Tubtfex there is only a single sac, such too is the case 

 with Mesenchytraeus. VEJDOVSKY (24) indeed seems to imply that unpaired sacs are 

 the rule, for he states in a footnote to p. 137 of his 'System und Morphologic' that 

 only in Rhynckelmis are the sperm-sacs, and, as we know, the egg-sacs, paired ; the 

 remark in the text to which this footnote is appended runs as follows : ' Sowohl 

 die Samen- als Eiersacke sind unpaarig,' &c. The paired condition, however, seems 

 to bo usual in the Lumbriculidae, for it is met with in Sutroa as well as in 

 Rhynchelmis ; the formation of the egg-sacs in the aquatic Oligochaeta seems to be 

 merely a pushing out of the septa, caused, perhaps, by purely mechanical reasons : 

 i. e. the large size of the ova and the pressure which they must consequently exert 

 as they are driven backwards by the development of those in front ; I have figured 

 in the genus Phreodrilua the first commencement of the egg-sac, where it is nothing 

 more than a bulging of the septum (21 PI. II, fig. 31). The egg-sacs were generally 

 termed 'ovaries' just as the sperm-sacs were called erroneously 'testes'; the excuse 

 for this error was in many cases the fact that the ovaries in the sexually mature 

 worm had entirely broken up into clumps of developing ova, leaving no trace of 

 their former existence. 



Among the terrestrial genera it is only in the family Moniligastridae that the 

 egg-sacs are at all large ; here they may extend through three or four segments ; 

 they are always paired sacs and enclose the numerous ripe ova ; as the Moniligas- 

 tridae are the only earthworms with comparatively large ova loaded with abundant 

 yolk, there almost appears to be some connexion between the large size of the 

 egg-sacs and this fact ; for in the aquatic Oligochaeta, also, the ova are loaded with 

 yolk spherules. 



The minute egg-sacs of Lumbricus were first discovered by HEKING who wrote 

 as follows : ' Der innere Rand des Trichters reicht an den Darmkanal ; am oberen 

 Rande ist ein kleines, durch das Septum in den vierzehnten Ring hineinragendes 

 Knb'tchen angewachsen. . . . Am oberen Rande (der Tuba) stiilpt sich die Wand 

 zu einem kurzen Fortsatz aus, der durch das Septum tritt und im vierzehnten 

 Segmente mit einer blaschenfbrmigen Erweiterung endigt. Dieses ist das erwahnte 

 Knb'tchen. Es zeigt eine sehr verschiedene Breite, durchschnittlich 0-5 mm., und 

 ist von einem dichten Gefassnetz umstrickt, dessen einzelne Gefasse bisweilen einen 

 Durchmesser von 0-03 mm. erreichen. Unter dem Mikroscop ist es schwierig, den 

 Zusammenhang dieses Organes mit der Tuba nachzuweisen, weil diese sich nicht von 

 dem muskulb'sen Septum ganzlich isoliren liisst, so dass jedes klare Bild unmoglich 



