RADIATA. 4] 
the latter are supposed to be shortened, and the concavity between them 
filled up, the approximation will appear when the correspondence of the 
organs is considered. 
The plates of an Hehinus (pl. 76, Vig. 69) run in vertical rows, two of 
which are wide and two narrow alternately; the wide pair have uButdlbe 
which support the larger spines; and the narrow ones have vertical rows 
of minute perforations which form the ambulacra, and allow the passage of 
the sucker-like feet which, in addition to the spines, are concerned in locomo- 
tion, and perhaps in passing water to the respiratory organs which lie beneath. 
The mouth of the Achinz is armed with five jaws working together by 
means of a complicated piece of mechanism, and which have been com- 
pared by Aristotle to a lantern, hence called the lantern of Aristotle. 
Among the chief authorities upon this department are Lamarck, Blain- 
ville, Delle Chiaje, Goldfuss, Desmoulins, J. Miller, Sars, Milne Edwards, 
Dufossé, Duvernoy, Klein, Gray, and especially Agassiz. 
The forms in this order are very various, from the flat and discoidal 
Scutella, which is flat beneath and but slightly convex above, to the sub- 
globular Echinus and elevated Galerites, which is considerably higher than 
wide. Some have the ambulacra disposed in oval or elliptic lines upon the 
upper surface, resembling the four or five petals of a spreading flower. 
According to Duvernoy, in the Echinidee in which the rosette is formed, a 
series of branchize (instead of feet) are passed through these perforations, in 
addition to the internal branchiz; and in consonance with this view, he 
divides the Echinide into two sections: the Exobranchia, with external 
branchiee (including forms like Clypeaster, Cassidulus, and Spatangus); and 
the Homopoda for the remainder (including forms like Cidaris and Galerites). 
LEchinus, and other genera with large spines, are found on the bottom of 
the sea, whilst the Scutel/w, which have short bristly spines, burrow in sand. 
Spatangus (pl. 76, jig. 67) and its allies have the mouth armed and 
placed towards the anterior end; the vent posterior, and placed upon the 
upper or lower surface; the shell thin in texture, lengthened, and gibbous ; 
ovarian pores four. 
Clypeaster (fig. 68), and the allied genera, have the mouth central, or 
nearly so, and the vent near the posterior margin, and upon the upper or 
lower surface, according to the genus. 
Echinus (pl. 16, fig. 69), and Cidaris (figs. 70, 71), have a subglobular 
shell and two kinds of spines, the larger of which are supported upon large 
tubercles. The mouth is central beneath, and the vent in the apex. 
The tendency to take an oblong form in Spatangus and Ananchytes, and 
the mouth being placed near the opposite extremities of the body, indicate 
an approach to the next order. 
Orver 4. HorornurtpEa. The animals composing this order have an 
elongated worm-like form, and the shell has disappeared, although some 
earthy matter is deposited around the mouth. In Holothuria and the allied 
genera the body is very contractile; the skin is irritable and has numerous 
mucous-secreting pores, and per Bbations for the passage of the sucker-like 
feet, which are either generally distributed, or arranged in five rows repre- 
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