72 ACALEPIIS IN GENERAL. Part I. 



between all Radiates, from the Erliinodenus down to the Polyps, without losing the 

 connection between their organic systems ; na}', even the form and special disposition 

 of certain organs in the two most remote classes of Radiates are intimately linked 

 together by peculiarities characteristic of some of the Acalephs. For instance, the 

 system of radiating tubes in the lower part of the disc of the genuine Medusa?, 

 and still more strikingly in the Rhizostomes and Cassiopeia?, presents the most 

 striking resemblance to the distribution of the ambulacral tubes in the lower wall 

 of the spherosome in Clypeaster, Scutella, and Echinarachnius ; so much so that 

 the difl'erence between the two types is reduced to the difference there is between 

 a soft wall and a solid wall, and that of an aminilacral system with and without 

 external suckers. But since there are Holothuria^ — the Synapta and allied genera 

 — in which these external suckers are wanting, the Avhole difference amounts only 

 to a different degree of complication in one and the same system, similar to the 

 various degrees of complication observed throughout the animal kingdom in the 

 differentiation of the organs. The comparisons I have been able to make between 

 Cassiopeia and Echinarachnius and Clypeaster are conclusive upon this point, as 

 will l)e shown in the sequel. 



After tracing so close a correspondence and so many connecting links in the 

 structure of the Echinoderms, Acalephs, and Polyps, I may be permitted to ask 

 what there is left to support the idea of a typical difference between the Echi- 

 nodermata and Coelenterata, now so generally and so strongly insisted upon by 

 German naturalists. The truth is that the Coelenterata do not constitute a primary- 

 division in the animal kingdom, Ijut must be united with the Echinoderms as 

 members of one and the same type, including three, and only three, natural classes, 

 equally distinct one from the other, — the Polyps, Acalephs, and Echlnoderms.^ 



I need hardly remind anatomists of the importance, for their own special studies, 

 attaching to every improvement in the classification of animals ; for it is only when 

 their natural affinities are satisfactorily known, that it is possible to give a compre- 

 hensive account of their structure. 



' If this be so, then the name of Ccelenternta diisina as inchuling the naked-eyed Medusa; with 



as designating a distinct t^ype, as well as tliat of tlieir polypoid congeners, must be dropped from 



Antliozoa as designating the Polyps in contra- tlie system of Zoology, and the older names Radi- 



distinction to the Hydroidea, and that of Hydrome- ata, Polypi, Acalephs, and Echinodermata, restored. 



