42 INTltODUCTION TO CLASSIFICATION. 



it. The two epipodia, on the other hand, unite posteriorly above 

 the foot, and where they coalesce, give rise either to a folded 

 muscular expansion, the edges of which are simply in apposition, 

 as in Nautilus; or to an elongated flexible tube, the apex of 

 which projects beyond the margin of the mantle (Fig. 17, /), 

 and is called the funnel or infundibulum, as in the dibranchiate 

 Cephalopoda. 



The Cephalopoda present a vast number of the most interest- 

 ing features, to which it would be necessary to devote much 

 attention if we were studying all the organic peculiarities mani- 

 fested by the class ; but it is in the characters of foot and of the 

 epipodium that the definition of the class must be chiefly sought. 

 In addition, the flexure of the intestine is, in all Cephalopods, 

 neural ; and the mouth is always provided with a horny or more 

 or less calcified beak, like that of a parrot, composed of two 

 curved pieces, which move in the median antero-posterior plane 

 of the body ; and one of which, that on the neural side, is always 

 longer than the other. 



XVI. THE ECHINODERMATA. 



The star-fishes, sea-urchins, sea-cucumbers, trepangs, and 

 feather-stars known technically as Asteridea, Echinidea, Holo- 

 ihuridea, Ophiuridea, Crinoidea, &c., are marine animals which 

 differ vastly in external appearance, though they all, in the 

 adult state, present a more or less definitely radiate arrange- 

 ment of some parts of their organization. 



That which most remarkably distinguishes the JEcliinoder- 

 mata is the nature of the embryo, and the strange character of 

 the process by which the adult form is originated by a secondary 

 development within that embryo. 



In the great majority* of the Echinodermata, the develop- 

 ment of which has been examined, the impregnated egg gives 

 rise to a free-swimming, ovoid, ciliated embryo, the cilia of 



* In Ophiolepis squamata and Echinaster sepositus, the larva appears to attain 

 only a very imperfect state of development before the appearance of the echinoderm 

 body ; and careful re-examination is required to decide how far the larvai of these 

 animals are truly bilaterally symmetrical. 



