Xll INTRODUCTION. 



while the first state of a Comatula is analogous to a sponge 

 or a Polypidom, the highest groups of Echinodermata are 

 creatures resembling Mollnsca or Annellida. Correspondent 

 with the progression of form is the progression of organiz- 

 ation and of sagacity. 



Externally, the majority of Echinodermata are radiated, 

 and the lowest groups resemble Polypes. The first species 

 figured in this history is the Feather-star, a creature which 

 in its youth is fixed and pedunculate, like a Zoophyte ; in 

 its adult state free and star-like. When dredging in Dub- 

 lin Bay in August 1840, with my friends Mr. R. Ball and 

 W. Thompson, we found numbers of the Phytocrinus or 

 Polype state of the Feather-star, more advanced than they 

 had ever been seen before, so advanced that we saw the 

 creature drop from its stem and swim about a true Co- 

 matula ; nor could we find any difference between it and 

 the perfect animal, when examining it under the micro- 

 scope. From the Comatula we proceed onwards through 

 forms gradually changing character, until in the Sea- 

 Urchins we have the true representatives of the Echino- 

 dermata. In them we have the perfection of an Echino- 

 dermatous integument. We have arrived, as it were, at 

 the summit of a pyramid, and we descend through a series 

 of forms as gradually conducting us to the Mollusca and 

 the Vermes. The Holothuriada?, which at first have the ap- 

 pearance of soft Sea-Urchins, gradually change their forms, 

 and become more and more Molluscan in character. The 

 Sipunculidae progress in like manner towards the Annel- 

 lida ; and in the animals described last in this volume, 

 we see Radiata, which have so put on the garb of worms 

 that by many naturalists they have been classed as such. 



Every great class in the animal kingdom, when con- 

 sidered anatomically or physiologically, may be looked on 



