474 



Marvels of the Universe 



Plio(o biq IE. J. Hinlla, F.R.M.H. 



SECTION OF SPINE. 



This magnified section of an Urchin's spine 

 presents an entirely different pattern from that 

 shown on page 472. 



be obtained in the sea around our own islands. In these 

 differences, which constitute distinct species, we find some 

 with shorter or longer spines, each with a constant and 

 characteristic grooving and internal pattern by which the 

 species may be distinguished. They differ also in colour, 

 some deep red, some green, others purple. Some, of which 

 the Heart-Urchin shown on page 473 may be taken as a 

 type, are clothed with silky bristles, all pointing one way, 

 instead of the spines, which point in all directions. These 

 are burrowing Urchins that live in sand or mud, and, in 

 consequence, are not so well known to the pubhc. These 

 will be seen to be oval instead of round, and as they live 

 upon substances that require no mastication they are 

 without teeth ; instead, the mouth is a simple open scoop 

 situated at the front of the lower surface, so that the 

 mere act of burrowing pushes the sand or mud into it, 

 and the nutritive material mixed up in the sand is then 

 digested out in the internal organs. 

 Another group comprises the Cake-Urchins and Shield-Urchins, which are very flat and thin. 

 Like the Common Urchin, they have the mouth in the centre of the underside, and it is provided 

 with apparatus for masticating the food ; but in the matter of spines they come nearer to the Heart- 

 Urchins. One of these is the well-known Sand Dollar, of North America, and the httle-known 

 Wheel-Urchin, from West Africa, which is represented by the lower photograph on page 473. The 

 specimens have been denuded of their spines. A remarkable point about the Wheel-Urchin is the 

 character which has suggested the name— the cog-hke indentations upon half the edge of the disc. 

 The pores through which the suckers emerge are all on the upper side, and they run off from the 

 little five-sided button in the centre like the petals of a flower. This button, by the way, should 

 be mentioned as one of the marvels of the Sea-Urchin and Star-fish organization. It is porous 

 and a natural filter. It is the intake for the water-vessels that work the thousands of sucker- 

 tubes. When one regards the minuteness of the pores in the walls through which these tubes 

 pass, it must be manifest that the freedom of the water filhng them from any particles of sohd 



matter is an absolute necessity for the working of the 

 system. This porous plate is Nature's plan for over- 

 coming that difficulty. No solid matter can pass through 

 it, and all the water for working the tubes does pass 

 through it and is kept separate from what passes through 

 the mouth. The Cake-Urchins having two almost flat 

 surfaces, require support for the roof, and so we find the 

 interior is strengthened by limy partitions connecting the 

 upper and lower surfaces. 



The Sea-Urchins are a very ancient family, and their 

 fossils are among the most abundant of the many forms 

 of life that have left evidence in the rocks of their former 

 existence. They appear as far back as Silurian times, for 

 fossil forms are found in the rocks formed at that period. 

 Certain beds of the Upper Chalk are full of fossil Sea- 

 Urchins. The fossils known as " fairy loaves " in chalk 

 districts are among the most plentiful of these, and until 

 recently that particular type of Urchin was supposed to be 



•ARISTOTLE'S LANTERN. 



The above name was given by the ancients 

 to the masticatory apparatus of the Sea-Urchin, 

 which consists of five strong teeth, of which 

 oniy the polished tips show outside the mouth. 



