CHAMBERS'S INFORMATION FOR THE PEOPLE. 



connect the various polypes together. Pennatula, 

 the Sea-pen, has one end of the axis, which is sup- 

 ported by a sclero-basic coral, fixed in the sand in 

 the sea-bottom. From this axis, lateral branches 

 are given off, upon which are placed the polypes. 

 Its colour is reddish yellow, and when irritated, it 

 shews phosphorescence. In Gorgonia (Sea-shrub), 

 the coral is branched, horny, and sclero-basic. 

 The Red Coral of commerce (Corallium rubrum), 

 whose sclero-basic coral is so much admired for 

 its fine colour and for ornamental purposes, has a 

 smooth and branched tree-like form, about one 

 foot in height, and is about the thickness of the 

 little finger. It is chiefly obtained from the Medi- 

 terranean, and is largely exported to India. 



ORDER 3. Ctenophora are free - swimming 

 ocean forms, which never develop a coral. They 

 are gelatinous-like bodies, spherical in form, very 

 delicate, and transparent Eight bands covered 

 with cilia run from pole to pole. By the motion of 

 these cilia, the animal moves along. A trace of a 

 nervous system has been discovered in some forms, 

 Beroe pileus, which is like a globe of jelly, about 

 half an inch in diameter, forms part of the food of 

 the whale, and is often seen in the English Channel. 



III. SUB-KINGDOM ECHINODERMATA. 



The Echinodermata (echinos, urchin, and derma, 

 skin), or Spiny-skinned Animals, are so called 

 because they have a crustaceous or coriaceous 

 covering generally armed with tubercles or spines. 

 They have their parts arranged radially, but they 

 also exhibit bilateral symmetry. The alimentary 

 canal is completely shut off from the body cavity. 

 The integument covering the animal is more or 

 less completely calcified by the deposition of cal- 

 careous particles. In these animals there exists a 

 peculiar system of tubes, called the ambulacral 

 system (ambulo, I walk), because they are used 

 tor the purpose of progression. The nervous 

 system consists of a gangliated nervous cord 

 surrounding the commencement of the gullet, and 

 sending branches which radiate outwards parallel 

 with the ambulacral tubes. A blood-vascular or 

 circulatory system is also present In their mode of 

 reproduction, the members of this group exhibit 

 some very peculiar phenomena. The embryo is 

 at first free, swimming, and ciliated, and is pro- 

 vided with distinct digestive organs, which do not 

 become converted into the corresponding struc- 

 tures of the adult This embryonic form has 

 been called a pseud-embryo (false-embryo), bear- 

 ing no resemblance to its parent; so much so, 

 that the embryo of Echinus was at one time 

 described as a distinct animal under the name of 

 Pluteus. Only a part of this embryo is converted 

 into the adult Echinoderm, the remainder entirely 

 disappearing. The adult form is, as it were, 

 a sprout from a particular part of this pseud- 

 embryo. This sprout goes on developing a 

 distinct mouth, digestive and other organs, until 

 it exactly resembles the adult form which gave 

 rise to the Pluteus. 



It contains several orders, two of which, the 

 Blastoidea and Cystoidea, are entirely extinct. 

 Representatives of these two orders, such as 

 Pentremites of the coal, and Echinospharites of 

 the Silurian rocks, flourished during the Palaeozoic 

 age. 



138 



ORDER i. The Crinoidea, or sea-lilies, are fixed 

 during the whole or part of their existence to the 

 sea-bottom by a jointed and flexible stalk. They 

 consist of a series of plates articulated together, 

 forming a central cup or disc. From the edge of 

 this cup there spring five arms, which bifurcate, 

 and thus form ten beautifully fringed arms. Run- 

 ning along the inside of the arms is a furrow, 

 covered in by the skin of the disc, and from which 

 are protruded the ambulacral feet. Amongst the 

 permanently fixed and best-known forms is the 

 Pentacrinus Captft-medusce, or Medusae -head 

 Star of the West Indies. More recently, several 



Pentacrinus Caput-medusse. 



new genera have been discovered, since deep-sea 

 exploration and dredging have been suggested and 

 carried out, such as Rhizocrinus, which was found 

 off the coast of Norway. In Comatula, or the 

 Feather-star, the adult form is free, whilst the 

 young, which was formerly described as a distinct 

 animal, is fixed. The Crinoidea, however, were 

 much more abundant during the Carboniferous 

 period, where the mountain limestone is largely 

 composed of the stalks of the forms that flourished 

 there. The individual joints of the stem, when 

 separated, are known as St Cuthbert's beads. In 

 the Muschelkalk, one of the secondary Triassic 

 rocks of Germany, a very pretty form, the 

 Encrinus liliformis, or stone-lily, is specially 

 abundant, and is very well known. 



ORDER 2. The Ophiuridea, represented by 

 the 'Sand-stars' (Ophiura) and 'Brittle-stars' 

 (Ophiocoma), are star-fishes having five slender 

 arms radiating from a small central disk, on the 

 lower surface of which the mouth is placed. The 

 arms do not contain any prolongation of the 

 viscera, but consist of a central series of ossicles 

 united by powerful muscles, and of an external 

 mailing of several series of plates of the perison. 

 The ambulacral vessel runs along the lower 

 surface of the arm, and gives off at each joint a 

 pair of long conical tubular appendages, without 

 suckers, which are used for locomotion and 

 respiration. There is no excretory opening, the 

 indigested matter being thrown out from the 

 mouth. 



ORDER 2. Asteroidea. The common Asterias, 

 or Star-fish, which may be taken as the type of 





