42 



LECTURE III. 



ON THE CLASSIFICATION OF ANIMALS. 



THE ECHINODEEMATA, SCOLECIDA, ANNELIDA, CRUSTACEA, 

 ARACHNIDA, MYRIAPODA, AND INSECTA. 



Hitherto, it lias not been a matter of very great difficulty to 

 discover the characters in which the members of the various 

 classes, which have passed under our notice, agree with one 

 another and differ from the members of all other classes. But 

 to-day we shall be met, at the outset of our studies, by a large 

 series of organisms which present us with much greater ob- 

 stacles, — the result, in a great measure, of imperfect knowledge. 



The first group on the list — the Echixodeemata — com- 

 prises the star-fishes, sea-urchins, sea-cucumbers, trepangs, and 

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

 thuridea, Opliiuridea, Crinoidea, &c, — 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 Eehinoder- 

 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- 



* In Opldolepis squarrn if a and Ecliinaster sejiositu*, 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 larvae of these 

 animals are truly bilaterally symmetrical. 



