SEA-URCHINS, STARFISH, AND BRITTLE-STARS. 139 



which lie on the inner side of the shell, and form very 

 conspicuous objects. Perhaps, however, in the common 

 urchin at least, you will be most struck by the apparent 

 emptiness of the shell. It contains a large amount of 

 watery perivisceral fluid, but even when the urchin is fully 

 ripe seems disproportionately large relatively to the con- 

 tained organs. It should be noticed that the shell is not 

 an external structure like the coat of a crab, for its outer 

 surface is covered by a thin layer of skin, and in develop- 

 ment it arises as an internal skeleton. The separate plates 

 of which it is composed go on growing during life, and in 

 this way the whole test increases in size as the urchin 

 grows older. 



These two urchins are the commonest of our regular 

 urchins, which are characterised by their more or less 

 spherical shape and the regular arrangement of their tube- 

 feet in five double rows. The majority of the internal 

 organs, reproductive organs, nerves, ambulacral canals, etc., 

 occur in fives; or, in other words, the symmetry is penta- 

 merous throughout. It is otherwise with the next urchin 

 to be considered, which has a less well developed ambulacral 

 system, and shows a tendency to lose this five-rayed symmetry 

 in favour of a bilateral arrangement. There are a number 

 of such irregular urchins, but the commonest is perhaps 

 Ecliinocardium cordatum, which shares with some of its 

 allies the popular name of heart-urchin. The heart-urchins 

 are most interesting animals, interesting both in themselves 

 and in their contrast with the common urchins. To get 

 Echinocardium in the living state one must be prepared to 

 risk a good deal in the way of wet feet. If the enthusiasm 

 of the naturalist rises above this objection, the next desi- 

 deratum is a strong spade not a toy, but the genuine 

 article borrowed from the gardener and a good low spring 

 tide. The last is in most cases essential. Then choose a 

 spot where the tide ebbs a long distance over sand which is 

 shown, by abundant worm-castings and mollusc shells, to be 

 suited to animal life, and begin work at the margin of the 

 water. It may be well to repeat warnings already given as 

 to the force of spring tides and the possible element of 

 danger in shore hunting at that period. In most cases the 

 tide rushes in over those long level flats, beloved of sand- 



