6 ECHINOIDEA. 



cated, calcareous, composed of thick plates closely articulated together, the number and 

 arrangement of which are determinate in the different famihes, the multiples of five being the 

 numbers which predominate ; the central plate of the body is supported on a long, jointed 

 column, which is firmly rooted to the sea-bottom. The mouth is central and prominent ; 

 the anus is situated at its side ; the arms are mostly ramose and multi-articulate, and when 

 extended form a net-like instrument of considerable dimensions. The mouth is always 

 placed upwards, and retained in that position by the column being jointed to the central 

 plate of the calyx. The normal station of the Crinoidea is the reverse, therefore, of the 



AsTEROIDEA and EUHINOIDEA. 



Type. Pentacrinus Caput-medusa. Miller. From the seas of the Antilles. 

 Extinct families of Crinoids have existed in all seas from the Silurian downwards, and 

 one or two representatives are now living. 



Erom the above analysis of the class Echinodermata, it appears that, as the Sipwncu- 

 loidea and Holothuroidea are not found in a fossil state, and the Blastoidea and Cystoidea 

 are special to the Palaeozoic period, our field of investigation in this Monograph is limited 

 to the EcniNoiDEA, Asteroidea, Ophiuroidea, and Crinoidea, which we now propose 

 to consider seriatim, commencing with the Echinoidea. 



Order — Echinoidea. 



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

 test or shell, composed of ten columns of large plates, the inter-amhiilacral areas : and ten 

 columns of small plates, tke amhulacral areas, which are separated from each other by ten 

 rows of holes, constituting the poriferous zones. The external surface of the plates is 

 studded with tubercles of different sizes, in the different families ; these are articulated with 

 the spines by a kind of moveable ball-and-socket joint : the spines are of various forms 

 and dimensions, and serve well to characterise the genera and 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 perfo- 

 rated for lodging the eyes. There are two great apertures in the test, one for the mouth, 

 and the other for the anus ; the relative position of these oral and anal apertures varies in 

 the different families, and forms an important character for their systematic classification. 



The mouth is sometimes armed with a complicated apparatus of jaws and teeth, but 

 sometimes it is edentulous. The internal organs of digestion consist of a pharynx, oesopha- 

 gus, stomach, and intestine, which winds round the interior of the shell, attached thereto by 

 a delicate mesentery, its surface, as well as that forming the lining membrane of the shell, 

 is covered with vibratilc cilia, the play of which causes currents of sea-water to traverse 



