ECHINOIDEA. 19 



Order — Echinoidea. 



The body is spheroidal, oval, depressed or discoidal, and enclosed in a calcareous 

 test or shell, composed of ten columns of large plates, the inter -ambulacra! areas, and ten 

 columns of small plates, the ambulacra! areas, separated from each other by ten rows of 

 holes, the poriferous zones. The external surface of the plates is studded with tubercles 

 of various sizes, in the different families ; to these the spines are moveably articulated by 

 ball-and-socket joints ; the spines are of various forms and dimensions, and serve well to 

 characterize the species. 



At the summit of the test is the apical disc, composed of five genital plates, perforated 

 for the passage of the ovarial and seminal canals, and five ocular plates, notched or 

 perforated for lodging the eyes. There are two great openings in the test, one for the 

 mouth and the other for the vent ; the relative position of these apertures varies in different 

 families, and forms an important character in their systematic classification. 



The mouth in some families is armed with a complicated apparatus of jaws and teeth, 

 in others it is edentulous. The internal organs of digestion consist of a pharynx, 

 oesophagus, stomach, and intestine, which winds round the interior of the test, attached 

 thereto by a delicate mesentery; its surface, as well as the lining membrane of the 

 shell, is covered with vibratile cilia ; these cause currents of water to traverse the 

 interior of the body, and perform an important part in the function of respiration; 

 the blood is circulated in arteries and veins, aided by a central pulsating organ or heart. 

 The five ovaries and testicles occupy the ambulacral divisions, and open externally through 

 holes in the genital plates. Locomotion is performed by the joint action of the 

 tubular retractile suckers and the spines. Many sea-urchins attach themselves to rocks 

 by their tubular feet, and some bury themselves in limestone and sandstone, or even 

 granitic rocks, by the abrading action of the spines. 



The nervous system consists, according to M. Van Beneden, of a circular cord, which 

 surrounds the entrance to the digestive organs, and sends branches into the divisions of 

 the body. Professor Agassiz, and the late Professor Edward Eorbes, regarded the organs 

 situated in the ocular plates as eyes, but M. Dujardin 1 denies them even a nervous 

 system. In the absence of more direct anatomical evidence on the point, the following 

 observation, related by M. Alcide d'Orbigny, 2 has an important bearing on the question, 

 and supports it affirmatively • 



Captain Ferdinand de Cande, who commanded the ■ Cleopatre' in the Chinese seas, 

 told M. d'Orbigny that he had captured, on the coast of that region, an urchin with long 

 spines, probably a Diadema, which he examined in a vessel of water, " I hastened to 



1 Lamarck, ' Animaux sans Vertebres,' 2nd ed., torn, iii, p. 200. 



2 ' Paleontologie Francaise, Terrain Cretace,' torn, vi, p. 12. 



