276 OLIGOCHAETA 



We have a fair knowledge of all these genera. In nearly all the genera of 

 this family, the dorsal setae are wanting in a variable number (varying for 

 the genus) of the anterior segments. This ' cephalization,' as it has been termed 

 by LANKESTER, is still more marked in Chaetogaster, where the ventral setae also 

 of some of the anterior segments are wanting, a condition also seen in Uncinais. 

 The setae themselves present a considerable range of variety in form, which is 

 partly illustrated on p. 7 in woodcut, fig. 2 b. The two most usual kinds of setae 

 are the capilliform and the uncinate ; the former, which are never found except in the 

 dorsal bundles, are thicker at the point of origin, and taper gradually to the free 

 extremity ; they are sometimes straight, sometimes slightly curved, and longer or 

 shorter ; in Bohemilla alone these setae are provided with a row of lateral serrations. 

 The uncinate setae are always found in the ventral bundles, and sometimes in the dorsal 

 also. They are shorter and stouter than the others, sigmoid in shape, with a central 

 swelling, and deeply cleft at the free extremity. These two kinds of setae are 

 connected by transitional types, of which there are two types: (i) longish, straight 

 setae, with a swelling at about the end of the second or third, and faintly bifid at 

 the extremity (e.g. in dorsal bundles of middle segments of No/is elinguis); (2) shorter, 

 straight setae, sharp at the extremity, near to which is a swelling (e.g. in dorsal 

 bundles of hinder segments of Nais elinguis) ; (3) straight, but with a bent, 

 sickle-shaped extremity (Chaetobranchus). These two latter are termed by BOURNE 

 ' spear-shaped setae.' Besides these, there are the genital setae, which have as yet been 

 only described in a very few forms ; they are sigmoid in shape, and not cleft at the 

 extremity. The number of setae in each bundle is usually more than two. 



In this group of Oligochaeta alone (with the exception of Branchiura, Hesperodrilus 

 branchiatus, and possibly of the doubtful Alma nilotica of T^H'B.KN'BEna = Digitibranchus 

 niloticus, LEV.) the body-wall is prolonged into branchial processes. These are, however, 

 found in only two genera, Dero and Chaetobranchus; in the former they consist of 

 a set of processes surrounding the anus, in the latter of delicate outgrowths in which 

 the dorsal setae are imbedded. A more minute description of these organs will be 

 found below. 



The nervous system of the Naidomorpha is in advance of that of Aeolosoma, and 

 conforms to the type characteristic of the Oligochaeta in general. The cerebral 

 ganglia are free of the epidermis, and lie in the first segment; they are manifestly 

 composed of two halves, each of which is, as a rule, furnished with a lateral lobe. 

 A circumpharyngeal commissure unites these ganglia with the ventral ganglionated 

 chain. 



The nephridia of the Naidomorpha often show a peculiarity which has been also 



