626 PEOFESSOE E. EAT LANKESTEE. 



itself being daily extended by the addition of segments to its 

 free extremity (see fig. 1^ a, PI. XXXIX). 



The Khabdopleura mirabilis of Sars does not quite 

 agree with the above description, since the polypide tubes are 

 for no part of their course recumbent, but spring directly from 

 the axis at right angles to it. It is exceedingly probable, 

 however, that this difference is one due to the nature of the' 

 surface upon which the Rhabdopleura is growing. The 

 Hh. mirabilis of Sars was taken by him in fragments growing 

 upon coarse sand — attached not to a continuous support, but 

 to one piece after another of coarse angular rock-particles. 

 The Lervik specimens agree in the character of their polyp- 

 tubes with Dr. Norman^s original Shetland specimens, with 

 which I have been enabled by his kindness to compare them. 

 The latter were attached to dead shells, and one of the finest 

 specimens which I dredged at Lervik was spread over the dead 

 shell of a large Pecten. There is no doubt then, that the 

 Lervik species is the Rh. Normani of AUman. 



I do not think that Sars has given sufficient reason to lead 

 to the conclusion that his Rh. mirabilis is anything more 

 than a variety of Hh. Normani, determined by the character 

 of its support. 



The tubarium of Rhabdopleura is formed from a transpa- 

 rent substance of horny consistency. When a colony is ex- 

 amined with the microscope it is found that the whole 

 tubarium is built up of separate segments. In the free up- 

 standing polypide tubes these segments have the form of 

 complete rings, segments of a cylinder cut at right angles to 

 its long axis, which, to use a homely comparison, resemble a 

 number of table-napkin-rings piled one on the top of the other. 

 Under favorable circumstances these rings can be detached 

 from one another. Each ring is the work of one period of 

 activity on the part of the buccal-shield of the polypide in- 

 habiting the tube, and the older the polypide the more nume- 

 rous will be the rings ' of the tube added from time to time to 

 the mouth of the tube (PI. XXXIX, fig. 1, y). 



On the other hand, the segments which constitute the 



