II MR. E. R. LANKESTER ON SOME LOWER ANNELIDS. 



, lations as in JHolosoma. A curious form, described by 0. Schmidt in his paper already 



alluded to* soenw also to belong to the group formed by Chatogaster, Parthenope, and 



KnUmama ; he I'njmea it and briefly describes it as Thymmoplea ktctuosa. in this worm 



Hm- integument contains the same vesicular bodies characteristic of the above genera, 

 :ui(! to such an extent as to give it a warty appearance ; there are a few bundles of delicate 

 hair-iik setm, and a short prostomium; but the description of Schmidt does not enter 

 into anatomical details, and the worm appears to be very minute and difficult to study. 



Tin anon ilous chaTognathous Sagitta may fairly be compared to Chatogaster, if it is 

 regarded B a w,< segmental form. In comparing Chatogaster with other low forms of 

 Am lida, the great diiliculty is presented by its specialized cephalic bristles. The only 

 ot hre case in which Mich bristles are present is Sagitta, and the larval forms of Polychete 

 Annelids. If we deprive Chcetogaster of its posterior segments, reducing it to a uniseg- 

 mental condition, a head and body, or a product of simple anterior and posterior growth, 

 its general morphology is not so very dissimilar to that of Sagitta, its integument, its 

 cephalic bristles, and nervous system agreeing, whilst the single abdominal pair of bristle- 

 bundles may represent the " fins " of Sagitta. Such a comparison is further assisted by 

 Blias Mecznikow's description of Cl/retosoma of Claparede, and a new form, Bhabdosorna, 

 in which he points out their resemblances to Sagitta. Sagitta cannot for a moment be 

 placed in close connexion with Chcetogaster in any natural classification; for Sagitta is 

 elaborately modified on a untie gmental plan*, whilst Chcetogaster is modified on a multi- 

 8i imcntal plan ; but that they may have had some common ancestor does not appear im- 

 probable. If annul at ion or segmentation is to be taken as a necessary character of the 

 class to which Sagitta is now referred, it cannot remain there, since it is not segmented; 

 such a dit1erencc, however, has not prevented the association of Dinophihis, Alaurina, 

 and Microstomim with the other Turbellaria which are unsegmented, nor of Trematoda 

 with Ce^toda. The retention of imperfectly developed zooids in a chain cannot be regarded 

 as a vei important differentiating character. The moveable bristles of Sagitta and the 

 character of its integument are what constitute its claim to association with Chaeto- 

 podous A nnelids ; and it is precisely in these particulars, as pointed out to me by 

 Professor Huxley, that it resembles Chcetogaster. Starting, then, from such a Turbel- 

 larian form as Dmophilus (in which segmentation is beginning to show itself) we have 

 on one side, very remotely placed perhaps, Sagitta ; nearer to each we have Chcetogaster, 

 connected by Parthenope (Ctenodrilvs) to JEolosoma, which leads on to Nais and the 



Lumbricoids, and is also connected to the Turbellarians by so simple a form as Tfoj- 

 9a n plea. 



As to the classification of the genera Chcetogaster, JParthenope, Thysanoplea, and 

 JEoli oma. the difficulties which they present are those always found with the solitary 



low r " members of a Group. They are now put with the Naids, from which certainly 

 ChMogatter differs as much as Nais does from Lumbrieus; but at present it is perhaps 

 best that they should be left together, as forming the lowest representatives of the 

 Cluetopodous Annelids. It is among them that we must look for the representatives of 

 the lost progenitors of the Cha?topoda. 



., 



secondary 



