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XXII. — On the Sexual Phase in certain of the Naididse. I. The Anatomy of Sexual 

 Individuals of the Genus Dero ; with Remarks on H&monais. II. The Genital 

 Organs in the Genus Slavina. By J. Stephenson, D.Sc., Professor of Zoology, 

 Government College, Lahore. 



(MS. received January 14, 1915. Read February 15, 1915. Issued separately May 11, 1915.) 



[Plate LXXX.] 



The Naididse are a family of small fresh-water Oligochseta, in which the usual 

 mode of reproduction is the asexual, by fission ; sexual forms are, in general, rare, 

 and in the majority of species, and in some whole genera (e.g. Slavina), they have 

 not so far been observed at all. Records of the genital organs are therefore of 

 interest ; and if they were available throughout the group it can hardly be doubted 

 that we should be able to judge better of the affinities of genera and species, and 

 consecpiently to improve our classification ; since the diagnoses of species and genera, 

 and the scheme of classification, depend at present to an unduly large extent on one 

 single set of characters, the form and distribution of the setae. 



The study of sexual individuals of the genus Dero has resulted also in the dis- 

 covery of the curious phenomenon of the atrophy of the alimentary canal at sexual 

 maturity ; this occurs also in Hsemonais (v. ant.) ; it has not hitherto been found 

 elsewhere in the Oligochseta, though it is known to occur (apparently in a less 

 extreme form) in a number of Polychsetes. 



I. The Anatomy of Sexual Individuals of the Genus Dero ; with 

 Remarks on H^monais. (Plate LXXX.) 



Sexual organs are already known in one or two species of the genus Dero, but 

 not in the species (limosa) with which the following account is mainly concerned. 



The specimens, a description of which is here given, were obtained from the duck- 

 pond in the Zoological Gardens, Lahore, in March 1914. The material contained 

 other worms, which were also the subject of examination ; and the Deros were not 

 noticed until about a fortnight afterwards, during which interval the tube had 

 remained on the laboratory table. About half a dozen specimens, all sexually 

 mature, were then obtained ; an examination of the setse and gills showed the species 

 to be limosa. 



The genital organs are shown in fig. 1. 



The clitellum includes the whole of segments vi and vii, and the greater part 

 of v ; ventrally, indeed, it includes the whole of v, since it reaches the level of the 



TRANS. ROY. SOC. EDIN., VOL. L, PART IV (NO. 22). Ill 



