ECHINIDEA. 505 



This calcification of areolar or simply fibrous tissue, by the 

 deposit of mineral substance, not in the meshes of areolae, 

 but in intimate union with the organic basis, is a condition 

 of much interest to the physiologist; for it presents us 

 with an example, even in this low grade of the animal 

 kingdom, of a process which seems to have an important 

 share in the formation and growth of bone, namely, in 

 the progressive calcification of the fibrous tissue of the 

 periosteum membrane covering the bone." ^ 



From their peculiarity of structure they may be said to 

 be almost imperishable. Their shells exist abundantly in 

 all our chalky cliffs, innumerable specimens of which may 

 be obtained, exhibiting the same wondrous forms and 

 characters as those which now frequent our shores. 



The Crinoidea, " Sea-lilies," — so called from the resem- 

 ,blance which many of them present to the lily, — were ex- 

 ceedingly abundant in former ages of the world ; and their 

 remains often form the great bulk of large masses of rock, 

 fig. 241. Tliese animals were all supported upon a long 

 stalk, at the extremity of which they floated in the waters 

 of those ancient seas, spreading their long arms in every 

 direction in search of the small animals which constituted 

 their food. Each of the arms, again, was feathered with 

 a double series of similarly jointed appendages ; so that 

 the number of separate calcareous pieces forming the 

 skeleton of one of these animals was most enormous. It 

 has been calculated that one species, the Pentacrinus 

 hriareus, must have been com>posed of at least 150,000 

 joints; and "as each joint," according to Dr. Carpenter, 

 "was furnished with at least two bundles of muscular 

 fibre, — one for its contraction, the other for its extension, 

 — we have 3,000 such in the body of a single Perda- 

 crinus — an amount of muscular apparatus far exceeding 

 any that has been elsewhere observed in the animal 

 creation." A furrow runs along the inside of the arms, 

 which is covered with a continuation of the skin of the 

 disc ; and from this the ambulacra are protruded, as in 

 other Echinodermata. 



In the family* of Ophiuridea, so called from the resem- 

 blance of their arms to a serpent's tail, (Gr. ophis, a snake, 



(1) Dr. Carpente/, Cyclopcedia of Anatomy and Physiology. 



