E CHI NODE RMS. 47 



tube-feet are protruded ; and if we should examine the whole of the 

 urchin, there would be found five of these pairs of rows, exactly the same 

 number as in the starfish, and as in that form, the mouth is at the centre 

 of these rows. Now were we to split the urchin between these pairs of 

 rows in the same way that one often peels an orange, and flatten it out, 

 the result would be that the urchin would be a starfish in shape. 



There are, however, other differences to be noted. The sea-urchin has 

 teeth, and these, like the parts of the shell, are five in number, all meeting 

 in the centre ; and it is interesting to note that in the way these grow and 

 in the way they sharpen each other they present a close resemblance to 

 the teeth of squirrels and other rodents, to be described later in this volume. 

 These teeth are supported in a bony framework, and from the shape of the 

 whole apparatus, and since the whole was first studied and described by 

 the old Greek father of zoology, it is called " Aristotle's lantern." 



The sea-urchins, like the starfish, pass through a metamorphosis in the 

 course of their development, and the changes in this case are even more 

 marked than in the other. The embryo is provided with several long and 

 slender arms, each supported by a delicate calcareous rod, and yet all these 

 are temporary. The young urchin grows out of the side of the body of 

 the embryo, and gradually all these long arms are absorbed. The purpose 

 of these and similar long spines in various animals is to increase the size 

 of the whole, so that the risk of being swallowed will be diminished. 



The sea-urchin (Fig. 41) is common to both our northern coasts and 

 those of Europe. It is blessed with an enormous scientific name, Strongy- 

 loeentrotus drobachiensis. It lives in comparatively shallow water and is 

 especially fond of rocky localities. In some places where the rock is soft 

 these and other urchins gradually wear nests for themselves — round, deep 

 hollows in the solid stone. How they manage it is still a mystery. It may 

 be that the spines are the means of this excavation, bat their tips never 

 show any great wear. " Constant dropping wears away the stone," and 

 these nests are probably not excavated by one, but by a long series of indi- 

 viduals, each wearing its part. 



Even more common on the shores of Europe than this form is the 

 common sea-urchin or sea-egg. This latter name refers not only to its 

 shape, but to the fact that it is edible, whence its scientific name, Echinus 

 esculentus. In other and especially in tropical species the spines may attain 

 an enormous development, being in length three or four times the diameter 

 of the disc. Again, they may be large and like slate-pencils, or short and 

 with little caps at the top. These spines are frequently cut across, and the 

 sections thus produced, with their intricate, lace-like structure, are great 

 favorites with microscopists. 



