DESCRIPTIONS OF GENERA AND SPECIES 315 



The term MesenchytraeiLS was first applied by Eisen to a few species, which were 

 grouped together as a subgenus of Enchytraeus. 



It was thus defined:— 'The spermatozoa, as long as they remain in the perivisceral cavity of 

 the body, or in the vesicle of the efferent duct, are not free, hut encysted or congregated into small 

 globules surrounded by a membrane. The supra-oesophageal ganglion is deeply divided in front, 

 but straight behind or nearly so, the emargination being very inconsiderable. The tube of the efferent 

 duct is unusually short and broad, never more than six or eight times longer than the vesicle 

 of the said organs.' 



It will be observed that the two last of these characters are retained in Michaelsen's definition, 

 of which the above is substantially a reprint. As to the spermatozoa, Michaelsen does not use 

 the point in his re-definition of the genus or in his revision of the Enchytraeidae (5). In an earlier 

 paper, howeyer, (upon Archienchytraeus mSbii) he remarks :— ' Bedeutende Abweiohungen kann ieh 

 von P. beumeri constatiren. Bei diesem Wurm zerfallen die Hoden in einzelne Zellgruppen. 

 Diese Zellgruppen sammeln sich in zwei Sacken, die von Dissepiment XI/XII gebildet werden und 

 rechts und links vom Darm in das XII. Segment hineinragen. In diesen Sacken maohen dann 

 die Zellgruppen ihre Entwicklung zu Spermatozoen durch. Soweit ich ihre Entwicklung verfolgen 

 konnte, ungefahr bis zu dem Stadium, welches von E. mSbii in III, 6. V gezeiohnet ist, bleiben 

 die einzelnen Gruppen von einem feinem Hautchen umschlossen, wie es auch Eisen fur die Mesenchy- 

 traeen, zu denen P. heumeri nach dem System dieses Autors geh6rt, feststellt.' 



The matter evidently needs further inquiry. Michaelsen added two other characters of 

 importance to the definition of the genus, viz. the presence of sperm- and egg-sacs, and the peculiar 

 form of the nephridia. Besides describing several new forms referable to this genus, Michaelsen 

 rescued a species (JV; fenestratus) from Eisen's genus Neoenchytraeus, and assigned it to Mesenchytraeus. 

 Eisen himself distinguished Mesenchytraeus from Neoenchytraeus partly on account of the form of 

 the brain , in the former, the posterior margin is straight or slightly concave ; in the latter, it is 

 convex ; now, in N. fenestratus, the hinder margin of the brain is, as Vaillant has pointed out 

 (6, p. 249), so little convex that the worm ought to be assigned to the genus Mesenchytraeus. 

 Michaelsen also drew attention to this similarity between two species, supposed by Eisen 

 to belong to different sub-genera ; he furthermore asserted from an examination of the species that 

 the nephridia have the form so characteristic of the genus Mesenchytraeus, a fact which is not apparent 

 in the figure given by Eisen (13, PI. xiv, fig. 35). The sperm-ducts, too, are short, as they are in 

 Mesenchytraeus. The identity of Levinsen's genus Analycus with Eisen's Mesenchytraeus seems 

 to me to have been rightly established by Michaelsen. This identification rests chiefly upon 

 the apparently similar form of the nephridia in the two genera. In addition there are other points 

 of similarity which are dealt with in the following pages. 



The main characters which mark this genus are given in the diagnosis ; this 

 diagnosis may now be expanded a little ; the brain has two pairs of muscles attached 

 to its posterior margin, one pair above, the other below. The nephridia are remarkable 

 for the fact that the voluminous part of the organ, lying behind the septum, is lobate ; 

 this is brought out in Eisen's figures, which are stated by Michaelsen to be, in 

 some particulars, inaccurate ; Eisen represents the tube as lost in a mass of 

 suiTounding tissue ; Michaelsen says that the tube is so much and so closely 

 coiled that there is but little o f the cellular sheath visible, ' dass hier die umhiillende 



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