300 WILLIAM A. IIASWELL. 



mented van Beneden's description of most of the internal 

 organs, described tlic nervous system for the first time^ and 

 pointed out the erroneous nature of van Beneden's conclu- 

 sions as to the animal's affinities. 



Since the date of Fcettinger's paper Histriobdella has 

 never been re-investigated, and a good many points in its 

 structure still remain obscure. 



The animal with which the present paper deals, though 

 without doubt nearly related to Histriobdella, differs from it 

 in a number of points of greater or less importance; and I 

 have signalised these differences by giving it a new generic 

 name — Stratiodrilus. The species I propose to name 

 S. tasmanicus. 



External Features: Movements. 



Like Histriobdella, Stratiodrilus is a very small animal, the 

 largest being onl}^ a little over a millimetre in length, and 

 about one sixth of a millimetre in greatest breadth. In general 

 shape (figs. 1 and 2) it approaches very near to Histriobdella. 

 In front is a well-marked head, separated by a constriction 

 from the body. The head is dorso-ventrally compressed, 

 convex dorsally, nearly flat ventrally ; in outline as seen from 

 above it is approximately heart-shaped — the apex, which is 

 rounded off, directed forwards. There is no distinction into 

 prostomiuni and peristomium, and the mouth is situated on 

 the ventral surface quite close to the anterior extremity. 

 The head bears seven appendages, arranged exactly in the 

 same way as the appendages of the head of Histriobdella, 

 but differing in shape. Five of these appendages are tactile, 

 and may be called tentacles. The other two aid in locomo- 

 tion, and may be termed the anterior limbs. Of the tentacles, 

 one (fig. 1, t^) is median, and projects forwards from a point 

 immediately above and behind the middle of the anterior 

 margin of the head; it is about one sixth of the length of the 

 head, very slender, cylindrical, and un jointed. The most 

 anterior pair of tentacles (/") are a little longer than the 



