780 PROFESSOR J. STEPHENSON 



The characters which distinguish the present species from H. ivaldvogeli, 

 previously described by Bretscher, appear to be the following : — The ventral setae 

 of segment viii are not here thicker than those of other segments, as Bretscher 

 found them in his species ; the shaft of the dorsal setae is considerably less curved 

 than in H. ivaldvogeli, and the prongs are here markedly unequal in length (contrast 

 with Bretscher's fig. 12) ; less important j)oints are the greater size (up to 20 mm.) 

 of the present species (H. ivaldvogeli 5-12 mm.), and the fact that the ventral setae 

 may be as many as four per bundle (H. ivaldvogeli 2-3 per bundle). I am uncertain 

 what value to attach to the difference in the position of the first nephridium (in 

 H. ivaldvogeli in segment iv, ix, or x, in the present species in vii). 



The remarkable features presented by these worms may be summarised as 

 follows : — 



(1) The variability of the setae ; the change in the character of the ventral setae 

 on passing backwards, and the fact that the change is established gradually, and at a 

 level much behind that which is usual in the Naididae. 



(2) The distribution of the dorsal setae, and the way in which the peculiarity comes 

 about — by the falling out of certain setal bundles in the anterior part of the body. 



(3) The distribution of chloragogen pigment, which extends on to the pharynx 

 and even reaches the prostomium. 



(4) The form of the pharyngeal cavity, and the pharyngeal gland cells. 



(5) The characters of the posterior intestine and the anus. 



(6) The presence of parietal capillary vessels, and the situation of the dorsal 

 vessel on the left side. 



(7) The atrophy of the alimentary canal in the fully mature sexual individual. 

 Bretscher points out the resemblance of the highly developed vascular system 



to that of Tubificids. Other Tubificid marks are seen in the comparative absence 

 of differentiation in the alimentary canal behind the pharynx ; and in the vertical 

 muscle of the setal bundles, which passes between the dorsal and ventral bundles of 

 the same side of a segment. But there is, of course, no doubt that the genus belongs 

 to the Naididae, in virtue of the occurrence of asexual multiplication and the 

 characteristic position of the gonads. 



Within the Naididae, the genera Dero, Aulophorus, and Branchiodrilus are re- 

 called by the position of the dorsal vessel and the presence of parietal capillaries. 

 The limitation of the dorsal setae is reminiscent of Ch&togaster, where the dorsal 

 bundles have entirely disappeared throughout the body ; there is, however, no reason 

 for assuming a direct relationship, and the comparison is of interest only as showing 

 an intermediate condition, which, if it were to progress, would eventuate in the total 

 absence of the dorsal setae which is characteristic of Chxtogaster. The closest 

 relationships of Haemonais are quite possibly to Nais itself, where five segments are 

 intercalated at the time of fission to form the head of the second animal, and where 

 (as presumably originally in Hwmonais) the dorsal setae begin on segment vi. 



