INTRODUCTION. XV 



Order IV. CiRRHi-SriNiGRADA. Echinidttc — Gradual disappear- 

 ance of lobes ; cirrhifcrous canals appearing as 

 avenues where cirrhi act as in Order III. but are 

 assisted by mobile spines clothing the integument. 

 V. Cirrhi-Vermigrada. Holothuriad^e — Lobes disap- 

 pear ; motions effected by avenues of cirrhi, assisted 

 by contraction and extension of the soft body. 

 VI. Vermigrada. SipunculiDvE — Cirrhi become obsolete 

 and disappear ; motion effected by the contraction 

 and extension of the animal's body. 



A glance at this arrangement will at once show that it 

 is most natural. There is nothing novel in it as regards 

 the constitution of the groups, saving the recognition of the 

 Ophiuridse as an order equivalent to the other orders ; but 

 as an explanation of the true nature and relation of the 

 Echinodermatous tribes, I prefer it to any arrangement at 

 present used, and have accordingly followed it through- 

 out this work. 



All the Radiata are greatly influenced in the arrangement 

 of their parts by some definite number. In the Echino- 

 dermata the reigning number is five. The name of " five- 

 fingers," commonly applied by mariners to the Starfishes, 

 is founded on a popular recognition of the number regnant. 

 It has long been noticed. Among the problems proposed 

 by that true-spirited but eccentric philosopher, Sir Thomas 

 Browne, is one, " Why, among Sea-stars, Nature chiefly 

 delighteth in five points V and in his Garden of Cyrus 

 he observes, " By the same number (five) doth Nature 

 divide the circle of the Sea-star, and in that order and 

 number disposeth those elegant semicircles or dental sockets 

 and eggs in the Sea Hedge-hog." Among the lower and 

 the typical orders we find this number regulating the num- 

 ber of parts. Every plate of the Sea-Urchin is built up of 



