272 J. T. CUNNINGHAM. 



life is homogeneous and semi-liquid. On the dorsal side of the 

 tube is a space between the gelatinous medulla and the wall of 

 the tube, and the edge of the medulla below this space is deeply 

 stained. All this is, in my opinion, the consequence of con- 

 traction and coagulation. 



Myxicola in the greater part of its body has a single tubular 

 fibre similar in structure to one of the pair which exist in 

 Sabella. According to Claparede (loc. cit ) the tubular fibre 

 of one side, at a point a little behind the oesophageal commis- 

 sures, opens into that of the other side, and the latter proceeds 

 as the unique fibre, while the nerve-cord corresponding to the 

 first tube joins the other cord without fusing with it and 

 finally terminates. My series of sections of this animal is not 

 quite perfect, but, as far as I can judge, Claparede has been 

 somewhat deceived. It is true that only one tubular fibre 

 persists, but it seems to me that the other terminates and does 

 not open into its fellow. Some distance behind the oesopha- 

 geal ring the right hand cord is seen thicker than the other 

 and destitute of a tubular fibre ; the tubular fibre is seen on 

 the left-hand side of the ventral median mesentery, which is 

 pushed considerably to the right. Farther back the tubular 

 fibre becomes central, and the two nerve-cords, oue on either 

 side of it, are equal in thickness. I have seen no indication of 

 the disappearance of one nerve-cord. In my opinion, all that 

 has happened in Myxicola is that one of the large tubes has 

 disappeared except in the extreme anterior region, and the 

 other has increased in size, and in the greater part of the body 

 become central. One very interesting point to be seen in 

 Myxicola is that the two tubes are continuous with one 

 another in the lower part of the cerebral commissure; 

 the tubes in each oesophageal commissure, which are of con- 

 siderable diameter, can be seen to become continuous with one 

 another in the section of the cerebral commissure. In my 

 sections of Myxicola the tube in the posterior part of the body 

 is entirely empty, except in the ventral part, where a thick 

 stained band occupies the cavity ; this is due to a greater 

 shrinking of the contents of the tube in the process of prepa- 



