438 REVIEWS. 



or body-chambers of Actinia, parts of the general cavity in these 

 animals, trnly correspond with the perivisceral cavity of the Echi- 

 noderms, within which the nutrient fluid circulates. And in stating 

 this conclusion, supported alike by the facts of structure and develop- 

 ment, we find our own opinions in complete accordance with those 

 entertained by all who have occupied themselves in any careful 

 degree with such considerations. The erroneous view formerly held 

 on this subject, and which unhappily yet lingers in some elementary 

 works, was altogether of an opposite character, mistaking, as it did, 

 the ambulacral apparatus of the Echinoderms for a true sanguineous 

 system. Parts really homologous to ambulacral vessels must be 

 sought, not among the structures of the Coelenterata, but in the 

 canals of the water- vascular system of the Eotifera and Worms. The 

 assertion that " the central cavity, in open communication with the 

 radiating chambers in Polyps, is closed in Acalephs, and communi- 

 cates only through narrow openings with the radiating tubes" con- 

 veys, by implication, an exaggerated view of what is, in truth, a very 

 feeble difference of degree ; all the parts here mentioned being, as we 

 have already said, but portions of the same great body-cavity. The 

 alimentary canal of an Echinoderm cannot, with strict propriety, be 

 considered homologous to any organ performing a like function 

 among the Coelenterata, seeing that its mode of development and rela- 

 tive position to the general cavity are so essentially dissimilar. 



The following is the arrangement of Eadiata adopted by Professor 

 Agassiz. The names of the several orders and classes, as we have 

 sought here to present them, follow one another in what he considers 

 to be their gradation, or relative rank, the highest appearing first in 

 the descending scale.* 



Sub-kingdom . . BADIATA. 



Class I.-ECHINODEEMATA. 

 Order 1. — Holothueoid^. 



„ 2. ECHINOIDJE. 



„ 3. — AsTERionxa;. 

 „ 4. — Ceinoid^i. 

 Class II.— ACALEPHS. 

 Order 1. — Ctenophoeje. 



„ 2. — DlSCOPHOEiE. 



„ 3. — HYDEOinjs. 

 Class III. -POLYPI. 



Order 1. — Halcto^oid^e. 

 „ 2.— AcTiNorcyE. 



As the Echinodermata are but incidentally alluded to in the 



* In compiling this table we have availed ourselves of the author's Essay on 

 Classification, in the first volume of these Contributions. 



