CHAPTER V11L 
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. ^} ie se a-cucumber—especially the three former—are well known to all 
frequenters of the seashore; while the fossil sea-urchins of the Chalk, whose Hint- 
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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