RADIATA. 41 



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 Echinus {pi. 76, Jig. 69) run in vertical rows, two of 

 which are wide and two narrow alternately; the wide pair have tubercles 

 which support the larger spines ; and the narrow ones have vertical rows 

 of minute perforations which form the ambulaora.^ 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 Echini 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. Midler, Sars, Milne Edwards, 

 Dufosse, Duvernoy, Klein, Gray, and especially Agassiz. 



The forms in this order are very various, from the flat and discoidal 

 Scutella., which is fiat beneath and but slightly convex above, to the sub- 

 globular Echinus and elevated Oalerites., which is considerably higher than 

 wide. Some have the ambulacra disjjosed in oval or elliptic lines upon the 

 upper surface, resembling the four or five petals of a spreading flower. 

 According to Duvernoy, in the Echinidte in which the rosette is formed, a 

 series of branchiae (instead of feet) are passed through these perforations, in 

 addition to the internal branchiiB ; and in consonance w^ith this view, he 

 divides the EchinidiE into two sections : the Exobranchia, with external 

 branchiae (including forms like Clypeaster, Cassidulus, and Spatangus); and 

 the Homopoda for the remainder (including forms like Cidaris and Galerites). 



Echinus., and other genera with large spines, are found on the bottom of 

 the sea, whilst the ScxitellcB., which have short bristly spines, burrow in sand. 



Spatangus {pi. 76, fig. 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 

 nearl}'- so, and the vent near the posterior margin, and upon the upper or 

 lower surface, according to the genus. 



Echinus {pi. 76, fig. 69), and Cidaris {figs. 70, 71), have a subglobular 

 shell and tM'o kinds of spines, the larger of whicli are supported upon large 

 tubercles. The mouth is central beneath, and the vent in the apex. 



The tendency to take an oblong form in Spatangics and Ananchi/fes, and 

 the mouth being placed near the opposite extremities of the body, indicate 

 an approach to the next order. 



Order 4. Holothuridea. 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 perforations for the passage of the sucker-like 

 feet, which are either generally distributed, or arranged in five rows repre- 



245 



