t 



324 



APPENDIX. 



sea-hcdoeJwo-^ so called from the spines that stand out in all direc- 

 tions over the thin, but firm, hollow shell ; third, the Holoihurians, 

 or -f-^vr-i-Z/z^i-, alluded to on page 129, whose bodies are long and 

 flexible, and the exterior is a fleshy skin, usually thick, often with 

 calcareous points or pieces in the skin, but not enough to interfere 

 with its slug-like flexibility. There are also other lower kinds, 

 which need not be here described. 



In Polyps the number of similar radiate parts in the structure is 

 typically a multiple either of six or oi foiij-j in Acalephs, oi four j 

 in Echinoderms, oi five. Some variations occur under each of these 

 divisions ; but they may probably be regarded as modifications of 

 the type by suppression in development, or the reverse. 



The Echinoderms are the highest of Radiates. They show their 

 superiority of rank in having more perfect nervous, digestive, and 

 branchial systems, generally an anal opening to the alimentary 

 canal instead of only a mouth, and a better organized mouth ; also 

 in the absence of lasso-cells, this provision of a stingmg apparatus 

 in the skin being a special attribute of inferiority. They have 

 tentacles (under the form of suckers and also of branchiae), but 

 these organs are usually arranged along the body radiately with 

 reference to the mouth or the opposite extremity of the animal ; 

 and the tentacular (or ambulacral) compartments alternate with 

 others non-tentacular (inter-ambulacral). When the body is long, 

 as in the Holothurians, the five ranges of tentacles extend along 

 the sides of the body. 



In many points, the Echinoderms are unlike Polyps ; and yet 

 the two are fundamentally similar in the radiate system at the basis 

 of the structure ; in the alternation of tentacular and non-tentacular 

 compartments when both kinds exist ; in the annular character of 

 the nervous system — for, although the nervous ring is not complete 

 either in Polyps or Acalephs, the isolated parts existing in these 

 species are manifestly rudiments of the nervous ring of the 

 Echinoderms ; in the system of water-circulation, which in Polyps 

 differs from that of Echinoderms only in being less perfect ; and in 

 other points which cannot here be dwelt upon. 



To the more scientific reader a word is here added on the question 

 whether Echinoderms are true Radiates. They have been separated 

 from this sub-kingdom by some zoologists on the ground of their 

 having a better defined alimentary canal, with two extremities to it 

 instead of only a mouth ; also a more perfect nervous system and 

 a more perfect aquiferous system ; and their not being furnished 

 with lasso-cells : — the Polyps and Acalephs being distinctively 

 designated by such systematists Ccclenterates. But the organs, or 

 arrangements, for the purposes of digestion, sensation, aeration, 

 prehension, are only the means by which the animal sustains itself 

 and does its work, while the type of structure is something fund- 

 amental to all these conditions of its exhibition. The fact of the 

 radiate structure, and of the general homology in the several parts 

 between the Echinoderms and other Radiates, is not affected by the 



