HOMOLOGIES OF ECHINODERMS. 89 



affinity, and has assumed the composite nature of Echinoderms, which he 

 considers as a colony of five persons united at the buccal extremity in a 

 somewhat similar way to that of a colony of compound Ascidians hav- 

 ing a common cloacal opening. 



He considers each arm of a Starfish, for 'nstance, — and it will apply 

 equally well to any Sea-urchin, — as made up of a series of distinct ar- 

 ticulations, just as well marked as the articulation of any Annelid. To 

 a certain extent this analogy is correct; we find a repetition of very 

 similar parts, a remarkable vegetative capacity in all Echinoderms. which 

 at first glance might seem to be of great importance as confirming their 

 articulate affinities. Yet the earliest stages of the young Echinoderms 

 in the Pluteus show beyond doubt that they have nothing in common 

 with a community. 



As w^ell might we compare the simple chymiferous tube of an Aca- 

 lepli with a single individual, and make a many-rayed Zygodactyla a 

 community of individuals with a single central digestive cavity. The 

 very fact that we can trace the passage between an Acalcph with a 

 polymeral chymiferous system like Zygodactyla and a Siphonophore zoid 

 in which we can trace but a single chymiferous tube, shows, at any 

 rate, that the number of ambulacral tubes should not be taken as any 

 proof whatever of a composite structure. When we come to the ar- 

 ticulation of the arms, can we consider that as anything beyond the 

 adaptation of the ambulacral system to the deposition of limestone plates, 

 allowing certain limited movements? In the whole order of Echinoderms 

 traces of this adaptation can be seen, as in some genera of Echini, 

 which, like Astropyga and the Armored Echini, retain Ji more or less 

 movable test, while in Holothurians the limestone deposition is reduced 

 to a minimum, the latter showing the range of a dermal and closed 

 ambulacral system, while the Ophiurans show the limit.-^ within which 

 the articulation can be developed in the Starfishes proper (Crinoids not 

 being in question at present). In consequence of this articulation and 

 their presumed Annulose affinities, Haeckle does not hesitate to derive 

 Echinoderms from w^orms, but as far as the orders of Echinoderms now 

 known are concerned, it seems impossible to imagine, even witii the 

 light palaBontology has thrown upon their appearance, how they have 

 succeeded one another, much less whence they have been derived. We 

 can readily see from the presence of several of the orders of Echino- 



