CHAPTER VIII 



STONE LILIES, STARFISHES, SEA URCHINS, AND SEA CUCUMBERS 

 Subkingdom ECHINODERMATA 



THE; starfish, the sea urchin, the brittle star, the feather star, and 

 Characteristics j. ne sea cucumber especially the three former are well known to all 



frequenters of the seashore: while the fossil sea urchins of the Chalk, 

 Group 



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 Mollusks, 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 starfish, or any of the animals named above, even a sea cucum- 

 ber 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 starfish it usually forms a kind of scaffolding, between which there stretches 

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

 sists 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 characteristic is due the name of the 

 group, derived from the Greek, echinos, a hedgehog, and derma, skin. Many ani- 

 mals have some deposit of lime, such as the shells of the Mollusks, and the bones 

 and teeth of the Vertebrates, but the deposit of the Echinodermata differs in 

 two characteristics: first, that its microscopic structure is that of a mesh work, or 

 rather or 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 crystallization. Owing to 

 these characteristics, the minutest portions of an echinoderm skeleton can be recog- 

 nized, even when fossilized. This tendency of the Echinoderms to deposit lime is 

 not confined to the skin, the walls of the internal organs being often strengthened 

 (3256) 



