RHABDOPLEURA MIRABILIS. 37 



but only very slowly, and usually only a little way, back into 

 its tube. This very slow and sluggish retraction, which may 

 last a very long time before it ceases, contrasts strongly 

 witli the extraordinary, almost lightning-like, rapidity with 

 wliich the retraction takes place in the other Polyzoa, and is 

 evidently accounted for by the want of special retractor 

 muscles, and by the slightly developed contractile elements, 

 not distinguishable as evident muscular fibres, in the con- 

 tractile cord, the only instrument by which the retraction of 

 the animal in the Rhabdopleiira is effected. 



The extension (protrusion) of the animal is yet far more 

 tardy than the retraction ; the process is extremely slow and 

 almost imperceptible ; several hours may often elapse before 

 the animal progresses from the stem or bottom of the cell to 

 the aperture of the latter. Neither do we, as before re- 

 marked, find in the animal under consideration the slightest 

 trace of any special muscles for such progression, since the 

 endocyst, and also the parietal and parieto-vaginal muscles 

 connected with it, are entirely wanting. The protrusion 

 seems, on the other hand, as already mentioned, to take place 

 in a very peculiar, and in the highest degree remarkable, 

 manner, that is, solely by means of the enormously developed 

 cpistome (buccal shield) which the animal uses, strange as 

 this may sound, as a sort of creeping organ, like the foot, or 

 creeping disc of the Gasteropods, to draw itself upwards, 

 little by little, along the wall of the cell to the aperture. 



The Rhabdopleura mirabilis seems to be a genuine deep- 

 sea product, which I have never found at a less depth than 

 100 fathoms ; but it is probably to be fovmd extensively at 

 greater depths, where it appears to be more and more 

 plentiful. I have hitherto only found it in Lofoten, where 

 it is not uncommon, in soft clay bottom, at depths of 100 to 

 300 fathoms. As the polyzoarium is both very small and 

 entirely colourless, it is rather difficult to discover. Its 

 presence is, however, easily detected by stirring the washed 

 mud in a fine sieve with a feather or other instrument, when 

 irregular fibres will be noticed therein. These fibres, 

 covered with particles of mud, Rhizopod-shells, and frag- 

 ments of mussel shells, will prove to be the creeping stem, 

 whereon, by closer investigation, there will be discovered the 

 small, transparent, perpendicularly projecting cells. One 

 seldom, however, succeeds in raising these colonies entire ; 

 they are most frequently broken into several pieces by the 

 dredging operation itself, or in washing out the mud. 



We may, according to the preceding description, charac- 

 terise the Genus Rhabdopleura in the following manner : 



