COMMON EGG-URCHIN. 



157 



indentation runs a raised stripe or rib longitudinally down- 

 wards through the flap. On the inner side of these flaps 

 at their base is seen an oval and apparently calcareous leaf. 

 The stem, which is similarly constituted with that of the 

 P. tridens, proceeds directly from the head (there is no 

 neck in this species), is small above, and thicker below, 

 until at the bottom it completely fills the hollow of the 

 sheath which encompasses it. With regard to the motions 

 of these Pedicellariee, they not only quickly open and shut 

 the three flaps, but can also turn the head to the different 

 sides, and up and down, and that very quickly. 



" Muller says, regarding P. tridens, ' Variat absque 

 aristis, an perditis \ ' Of such I have also found a large 

 number of specimens ; but I scarcely believe that they 

 belong to the P. tridens, since the teeth of these last are 

 fixed so firmly that they could scarcely fall off. Either 

 they are a separate species, or a variety of the P. triphylla, 

 which they resemble in every point except that the three 

 flaps are broad at the bottom and small at the ends. 

 These flaps seemed there also to be calcareous, and con- 

 sisted of many small globules, which were arranged in 

 transverse rows, clearly separated from each other by a 

 light transparent line. Such a line also ran longitudinally 

 down the flap. In P. triphylla the flaps are not obtuse, 

 but a little rounded, and have, like the foregoing, globules 

 extending in rows. 



" If we now consider the construction of the Pedicel- 

 larise, and their manner of life as a whole, we can scarcely 

 believe them to be anything but organs of the Sea-Urchin. 

 The following reasons seem to prove the accuracy of this 

 opinion. 



" 1st. In all Sea-Urchins without exception are found 

 Pedicellarite, and under the same circumstances, which 



