376 APPENDIX. 



Jive-fingers, whose bodies, although containing calcareous plates, are some- 

 what flexible, and ordinarily either five-rayed (fingered) or five-angled 

 (but sometimes more than five) : — second, the Echinus or sea-hedgehog, so 

 called from the spines that stand out in all directions over the thin, but 

 firm, hollow shell ; third, the Holothurians, or sea-slugs, alluded to on 

 page 160, 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 typi- 

 cally a multiple either of six or of four / in Acalephs, of four • in Echino- 

 derms, of five. Some variations occur under each of these divisions ; but 

 they may probably be regarded as modifications of the type by suppres- 

 sion in development, or the reverse. 



The Echinoderms are the highest of Radiates. They show their su- 

 periority 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 stinging apparatus in the skin being a special at- 

 tribute 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 an- 

 imal ; and the tentacular (or ambulacra!) compartments alternate with others 

 non-tentacular (inter- ambulacral). When the body is long, as in the Holo- 

 thurians, the five ranges of tentacles extend along the sides of the body. 



In many points, the Echinoderms ai-e unlike Polyps ; and yet the two 

 are fundamentally similar in the radiate system at the basis of the struc- 

 ture ; 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 Aca- 

 lephs, the isolated parts existing in these species are manifestly rudiments 

 of the nervous ring of the Echinoderms ; in the system of water-circula- 

 tion, 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 Padiates. 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 

 Ccelenterates. 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 



