772 



PROFESSOR J. STEPHENSON 



in the inner seta. This is shown in text-fig. 2, and the relative position of the bundles 

 of opposite sides in text-fig. 3. I have described (3) a similar condition in the form 

 which Michaelsen (2) has named Aulophorus stephensoni ; and it occurs also in Dero 

 limosa and Aulophorus furcatus. Since it is possible that it may occur more widely, 

 it is evidently not sufficient, in describing the setae of the Naididse (on the minutiae 

 of which the discrimination of species sometimes depends), to note the position of the 



Text-fig. 2. 



Text-fig. 3. 



nodulus in a single arbitrarily selected seta of a bundle without reference to the 

 condition in its fellows. 



When an individual belonging to the Naididse divides asexually, the zone of 

 production of new segments (budding zone) in the middle of the animal's body splits 

 in such a manner that a number of the new segments go to form the posterior end 

 of one daughter individual, while the rest form the anterior end of the other. This 



DucLeLjjyq Long. 



U VVi- 



B 



Text-fig. 4. 



latter group is commonly five in number (cf. Stephenson, 4), and it often happens 

 that these five newly formed segments, constituting the head of the posterior zooid, 

 bear no dorsal setae. Hence in a number of genera the dorsal setae commence on the 

 sixth segment. 



The above sequence of events takes place in the present species also ; and it 

 would accordingly be expected that the dorsal setae should begin, as in Nais and 

 some other genera, on the sixth segment. As a matter of fact they begin on or near 

 the twentieth ; and it is obvious, therefore, that in order to bring about this condition 

 a considerable number of segments must shed their dorsal setae about the time of the 

 detachment of the posterior zooid. Text-fig. 4 will make this clear ; it illustrates 



