CHAPTER VIII. 



STONE-LILIES, STAR-FISHES, SEA-URCHINS, AND SEA-CUCUMBERS, 

 Subkingdom ECHINODERMATA. 



Characteristics THE star-fish, the sea-urchin, the brittle-star, the feather-star, and 

 of the Group. ^j ie sea-cucumber especially the three former are well known to all 

 frequenters of the seashore; while the fossil sea-urchins of the Chalk, whose flint- 

 casts are so common on the downs of England, the so-called screw-stones found 

 in the Mountain Limestone, the pentremites and crinoids, whose remains are so 

 abundant in some parts of North America, are no less familiar to dwellers inland. 

 Though these animals differ much from one another in shape, a slight scrutiny 

 will discover many points in which they resemble one another and differ from 

 other creatures. They and their relatives are, therefore, placed in one great group 

 of -the animal kingdom, the Echinodermata, a group corresponding in importance 

 to the Molluscs, or the Vertebrates. This group is, in fact, more clearly defined, 

 and more widely removed from other groups than either of the two mentioned. 

 If a star-fish, or any of the animals named above, even a sea-cucumber or 

 holothurian, be touched with the finger, its skin will be found to have a rough 

 surface ; this is due to the circumstance that it contains a crystalline deposit 

 of carbonate of lime. In a sea-urchin, a brittle-star, or a feather-star, this deposit 

 is in the form of little plates, which build up a more or less rigid test ; whereas in 

 the star-fish it usually forms a kind of scaffolding, between which there stretches 

 the more yielding, leathery skin. In the ordinary sea-cucumbers the deposit 

 consists only of small spicules, which roughen the outer surface, and grate when 

 the skin is cut with a knife. If a thin slice of the skin of one of these animals be 

 cut and examined under a microscope, the spicules may easily be seen lying in 

 its middle layer. It is this same deposit that forms the spines of a sea-urchin and 

 the stalked column of a crinoid ; and it is this which has enabled so many of the 

 Echinodermata to be beautifully preserved as fossils. To this character is due the 

 name of the group, derived from the Greek, echinos, a hedgehog, and derma, skin. 

 Many animals have some deposit of lime, such as the shells of the Molluscs, and 

 the bones and teeth of the Vertebrates, but the deposit of the Echinodermata 

 differs in two characters : first, that its microscopic structure is that of a mesh- 

 work, or rather of a beam-and-rafter work, since it is deposited in the spaces 

 of a network of soft tissue ; secondly, that each element, whether a spicule or 

 a plate, is, despite its trellised structure, deposited around regular lines of 

 crystallisation. Owing to these characters, the minutest portions of an 

 echinoderm skeleton can be recognised, even when fossilised. This tendency of 

 the Echinoderms to deposit lime is not confined to the skin, the walls of the 

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