ON HAPLOBEANCHUS. 169 



worms without getting well besmeared, and sometimes walking 

 knee-deep in the tenaceous mire, it is hardly to be wondered at 

 that they have hitherto escaped attention/^ 



Associated with the worm was a species of Nais, which Pro- 

 fessor Lankester has identified as the Nais littoralis of O. 

 F. Miiller, which has for many years been unrecorded, no 

 naturalist, in fact, having seen it since Oersted's description 

 and figure in 184<2. There were also numerous free-living 

 nematoids and rhabdocoel planarians. 



Mr. Bolton tells me that he has seen the worm once before 

 in a gathering from the mouth of the LifFey. 



Anatomy. — The animal is minute, adult specimens not ex- 

 ceeding 6 mm. in length. 



The tube is about twice the length of the animal, and is 

 composed of particles of mud, with here and there a diatom 

 (Pleurosigma, sp.). 



Segments and appendages. — The '' head'' consists of a 

 prostomium and a peristomium.^ 



The prostomium is much reduced and hidden by the peris- 

 tomium, which rises to form a '' collar" around it ; this collar 

 is higher upon the ventral than upon the dorsal surface. There 

 are two prostomial tentacles, which are short, have pigment in 

 the walls, and are not ciliated ; they are much obscured by the 

 palps and peristomial tentacles (figs. 1, 2, 3, and 5, prost. 



1 In the description, I make use of the following nomenclature ; — The 

 Prostomium (Prsestomium, Huxley) is the lobe in front of the mouth; it 

 may bear two kinds of appendages, (1) prostomial tentacles (antennas, 

 Milne-Edwards and De Quatrefages ; cirri, Kinberg), which spring from 

 its dorsal surface, and generally resemble in character the appendages of 

 the peristomial somite; and (2) palps (palpi, Kinberg; infero-lateral 

 prsestomialcirri, Huxley; antennes externes, Milne-Edwards and 

 De Quatrefages), wliicli spring from the lower surface of the prostomium and 

 differ very considerably in character in different families of annelids. The 

 Peristomium (Peristomium, Huxley; Mund-segment, Grube), which is 

 the first somite of the body, and, it may be, the second and third fused with it ; 

 and although retaining ordinary characters in a few families of annelids, e.g. 

 Syllidae, very generally becomes much modified. Its appendages are peri- 

 stomial tentacles (tentacules, De Quatrefages), 



