200 L. Agassiz on the Natural Belations between 



tigator. And the necessity of combining the intestinal para- 

 sitic worms into one great natural group with the other 

 external free worms is becoming daily more evident to all, 

 so that whatever position be assigned to Annelides in the 

 great type of Articulata, Helminths have to follow them, and 

 must therefore be removed from the type of Kadiata. The 

 point is undisputed now, though there may be a difference of 

 opinion as to the propriety of admitting, to one great class, 

 all worms, or of subdividing them into minor natural groups. 



The third class among Radiata is that of Echinoderms, 

 which has been circumscribed within most natural limits 

 since the reunion of Holothurise and Crinoids, with the com- 

 mon star-fishes and true Echini. Whoever is familiar with 

 the embryonic development of Echinoderms, which has been 

 extensively investigated of late, will acknowledge an intimate 

 relation between them and the other two classes of Radiata, 

 and not be willing to assent to the proposed separation of 

 Echinoderms as one great type in the animal kingdom, placed 

 upon an equal footing with Mollusca, and will consider their 

 separation from Polypi and Medusae, as proposed by Dr 

 Leuckardt, rather as a retrograde step, than an improve- 

 ment upon the general classification of animals. To me the 

 t3rpe of Radiata, embracing the three classes of Echinoderms, 

 Medusse, and Polypi, constitutes, in its circumscription, illus- 

 trated above, a most natural group of the animal kingdom, 

 all the members of which are intimately connected by a close 

 uniformity in the plan of their structure, but present a re- 

 markable gradation of their types in the manner in which 

 this structure is developed in each of their classes. And 

 the circumstance that even in the higher ones, which con- 

 tain chiefly free movable animals, we have some few repre- 

 sentatives attached permanently to the soil, upon a Polypi- 

 like stalk, bearing the radiated animal crown, shews further 

 the intimate connection which exists between them all. Radi- 

 ata consist, therefore, of three classes only, which in their 

 natural gradation rank as follows : — Polypi, lowest ; next 

 Medusae ; and highest, Echinoderms. 



As soon as we have removed in this way all the classes or 

 families which do not strictly belong to the type of Radiata, 



