i] STRUCTURAL AND SYSTEMATIC 19 



sub-genera not showing any great divergencies from 

 the range of structure indicated in briefly defining 

 those enumerated above. 



Finally, we have the largest of all the sub-families 

 of this family, viz. that of the Megascolecinae. These 

 worms are mainly tropical in range and also mainly 

 found in the Indo-Australian portion of the world. 

 In them the sperm ducts open in common with 

 the usually single pair of spermiducal glands and 

 prevalently upon the xvinth segment. The glands 

 moreover have not always, though they often have, 

 the tubular form shown in all the sub-families hither- 

 to considered. In many forms they are branched 

 and lobate glands, and if there are two pairs one may 

 be of one type and the other of the second and 

 derived type, as for instance in Megascolex ceylonicus. 

 Furthermore, it is much commoner among the genera 

 of this sub-family for the setae to become numerous 

 and to spread right round the segment ; this con- 

 dition is seen in the genera Pheretima, Megascolex, 

 Diporochaeta, Perionyx, Plionogaster. The sper- 

 mathecae also are commonly more than the typical 

 two pairs of the forms already considered, and in 

 certain species (for example Pheretima hexatheca) 

 there are as many as six pairs of those organs which 

 are moreover and in this they resemble the majority 

 of species of the last subfamilies nearly always 

 furnished with a diverticulum or diverticula. The 



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