260 



NATURAL HISTORY. 



Genera. 



Astropecten, Solaster, Goniaster. 

 Ophiura, Ophiocoma, Ophiothrix. 

 Astrophyton, Gorgonocephalus, Em-yak.. 

 faltechinus, Archceoeidaris, Mi-Ion it vs. 

 ECHINOZOA { i?_i_i___-j__ Desmosticha . Echinus, Cidaris, Asthenosonia. 



Clypeaster, Mellita, Echinocyamuss. 

 Spatangus, Brissus, Pourtalesia- 

 Synapta, Chirodota, Molpadia. 

 Holothuria, Cucumaria, Psolus 

 Elpidia, Deiina, Ilyodasmon. 



inns, Cyathocrinus, Platycrinus. 

 Pentacrinus, Comatula, Apiocriwis. 

 PELMATOZOA { Cystoidea . . Glyptospharitcs, Echinospharites, Cnry- 



ocrinus. 



\ Blastoidea . . Penlremites, Orinatoerinvs. Cadaster. 



The names of orders and genera which are extinct are printed in italics. 



Exclusively marine animals, the Echinoderms remove limestone from its solution ir. the sea- 

 water, and build it up into a skeleton of very varied shape and of very different degrees of complexity. 

 This skeleton is least developed in the Sea-cucumbers, or Holothuroidea^ (Fig. 17). The skin of 

 these animals is very tough and leathery, with little limestone plates scattered about on it. Occasion- 

 ally, however, the plates are more developed, and overlap one another so as to form a continuous 

 covering all over the body (Psolus). Certain Sea-urchins (Echinoidea^) are also in the same condition, 

 the body retaining its flexibility, but being at the same time protected from injury by its coat of mail 

 (Asthenosoma\\). In most of the Urchins, however, the body is enclosed in a shell, or "test," which 

 is composed of numerous limestone plates, firmly united to one another by their edges, and supporting 

 spines of the same substance (Figs. 14 16). Besides this external skeleton, there is also more or less 

 of an internal skeleton, in the form of arched plates, pillars, or radiating partitions within the test. 

 Five pairs of arched plates, which are known as auriculae 9 ^ (Fig. 15), are of especial importance, as. 

 they occupy a very definite position with regard to the vascular trunks that radiate from the oral 

 centre. In the Starfishes and Brittle-stars they are represented by a double series of more or less. 

 arched limestone pieces, which form an internal skeleton in each arm, and are called the ambulacr.il 

 ossicles (Figs. 9, 13, ao). The skeleton of an Urchin, then, is almost entirely external, while that of 

 the Starfishes and Brittle-stars is chiefly internal ; but in both these classes, and especially in the latter, 

 there is also an external skeleton of limestone plates, -\\hich bears spines, and is sometimes very 

 considerably developed (Figs. 9, 11). Yet another form of skeleton is met with among the Crinoids.** 

 The successive joints which make up the arms of a Sea-lily (Fig. 18 ; Fig. 20, r.), although practically 

 external, are of a different nature from the pieces forming the test of an Urchin (Fig. 14), for they 

 occupy an exactly contrary position with respect to the vascular systems. Neither do they correspond 

 to the ambulacra! ossicles of a Starfish-arm (Fig. 9, ao), although these, together with the test of the 

 Urchin, are not altogether unrepresented in the Crinoids. 



The digestive tube of all Echinoderms is distinct from the general cavity of the body. It rmiy 

 be of considerable relative length, and make complicated windings within this cavity, as in the 

 Urchins, Holothurians, and Crinoids (Fig. 1 6, i ; Fig. 20, G) ; or it may be a short bag, without any 

 other opening than the wide mouth, as in the Ophiuridsff (Fig. 12) ; or lastly, there may be a short 

 and straight tube in the vertical axis of the body, with lateral extensions into the arms (Fig. 9, pc), as 

 in the Starfishes. 



The nervous system consists of an oral ring (Figs. 12, 20, ?zr), from which radiating cords 

 proceed along the primary divisions of the body (Figs. 9, 12, 13, 21, n). Both the ring and the 

 radial nerves originating in it are in very close relation to the celluiar covering of the oral surface of 

 the body, which is specially modified where the nervous tissue underlies it. This tissue consists of 

 closely packed fibrils and minute cells connected with them ; but there is no special arrangement of 



* For convenience of reference these two groups together are often spoken of as the Stellerida. 



t A good classification of the Asteroidea is still a desideratum. 



X Greek, holothourion ; cidos, form. H" Latin, diminutive of auris, ear. 



Greek, cchinos, hedgehog ; cidos, form. ** Greek, knnon, a lily ; cidos, form. 



|| Greek, asthenes, weak ; soma, body. tt Greek, ophis, a snake ; oitra, a tail ; eidos. form. 



