AND ANOTHER ALLIED ORGANISM. 55 



I have seen what he describes. He also draws and gives an 

 account of a protuberance about midway in the length of the body, 

 " at the top of which another orifice opens in a transverse direction 

 in the shape of a button-hole." He continues, " I have never been 

 able certainly to ascertain, although all my attention has been 

 fixed on this point, whether this slit communicated with the 

 central cavity. I cannot, therefore, say whether it ought to be 

 considered an anus." I have never made out this protuberance, 

 as clearly as M. Duthiers appears to have done, and think that 

 the slit described may possibly be an optical illusion, produced by 

 seeing somewhat askew the central organ, which I have drawn on 

 an enlarged scale, from a favourable specimen which I took special 

 care in examining. This organ had puzzled me much, and it is 

 the only portion of the Bucephalus which has given me the im- 

 pression of its being a sucker. It is usually seen tilted in relation 

 to the long axis of the body, and I have fancied the oesophageal 

 tube, which is easily traced to this point in well developed 

 specimens, passed through the centre. His description of a 

 central digestive cavity, which M. Giard calls in question, I have 

 also never recognised, but have often seen what appeared like a 

 vacant space of considerable extent in the centre of the body at a 

 comparatively early stage of its development. The long fila- 

 mentous appendages, which are finely striated throughout their 

 length, much like voluntary muscular fibre, always vary in 

 calibre when extended, being thickest at their attachment, and 

 diminishing toward their free end, the striation varying in close- 

 ness with the amount of extension. It is only in an early stage that 

 these arms appear of equal calibre tbroughout, and then they are very 

 obscurely striated. Beyond the origin of these filaments there is a 

 peculiar structure, darker in colour, and looking like two pads, 

 uniting in the long axis of the body. This is only seen, as in the 

 diagram, when the organism is as fully matured as I have ever 

 observed it. M. Duthiers appears to me to have drawn this 

 portion from an immature specimen. These pads, when examined 

 by polarized light, exhibited appearances which made me think 

 that here might be the beginning of rudimentary shells. The 

 organism found in the mussel is, as you see, chiefly different from 

 B. Haimeanus, in the form of its lamellar appendages, but here 

 the difference is very great in all stages of its development, and 

 when matured the contrast is very remarkable, the filaments, 



