SERPUL^ — SPIROBBIS. 421 



made in three stories, each smaller than that below it, so that it 

 bears a distant resemblance to the fusee of a watch. In the Brit- 

 ish Museum is a fine example of this genus Pomatostegus Bower- 

 hanlcii), in which the operculum rises li^e the conventional tower 

 of Babel, all the stories being devoid of horns, but covered with 

 short hairs of a fibrous nature. If the reader should happen to 

 be acquainted with conchology, he can form a very good idea of 

 this remarkable operculum by taking up the similar organ in 

 any species of solarium, or staircase shell, except that in the Ser- 

 pula the stories of the operculum are distinct, and not formed by 

 successive whorls. 



The operculum of the animal which is supposed to be under 

 examination does not present any of these singular appendages, 

 but is more or less conical, grooved above in a radiated form, and 

 horny in substance. It is, however, a very beautiful object, if 

 only for the elegant shape, which remains after the softer parts 

 have perished, and is in form so like a wine-glass with beautifully 

 fluted sides, that Dr. Johnston has remarked that jt might serve 

 as a pattern for that article. 



There is another marine annelid which constructs calcareous 

 tubes, and which is sufficiently interesting to warrant a short no- 

 tice. If the reader should happen to have in his possession a 

 piece of stone, or an oyster-shell, that has been for some time im- 

 mersed in the sea, or if he has a piece of the large tangleweed 

 that is so popular as a barometer, he will see that upon the sur- 

 face are certain tiny calcareous tubes, scarcely thicker than hairs, 

 and rolled spirally so as to form flat circular objects about as 

 large as pins' heads. They are firmly fixed to the objects on 

 which they are placed, and are often thought to be the earlier 

 forms of the serpula. 



These, however, are distinct animals, called Spirorbis by natu- 

 ralists, who have noticed that their spiral tubes bear a great re- 

 semblance to the planorbis shell, which is so plentiful in our riv- 

 ers and ponds. A tolerably powerful magnifying-glass is needed 

 before the real nature of the Spirorbis can be made out; but a 

 short examination will show that not only is the little worm fur- 

 nished with gills, or branchise which closely resemble those or- 

 gans in the serpula, but that, like that animal, it can shut up its 

 tube with an operculum of a conical shape. 



