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ECHINODERMA TA. 



CHAPTER XVII. 

 ECHINODERMA TA. 



THE Echinodermata, including the Sea-urchins, Star-fishes, Sea- 

 cucumbers, &c., form a very distinctly circumscribed group of 

 the animal kingdom, and were formerly included in the old 

 sub-kingdom Radiata. To Professor Huxley is due the credit 

 of having first pointed out that the Echinoderms possess cer- 

 tain remarkable affinities with the lower Worms, and especially 

 with those " Scolecids " which constitute the order of the Turbel- 

 laria. So well marked are these affinities that the above-men- 

 tioned eminent zoologist at one time proposed to unite the 

 Echinodermata with the Scolecida, to form a common division, 

 or sub-kingdom, under the name of Annuloida; and there are 

 many aspects in which this arrangement presents itself as a 

 highly convenient one. The progress of modern Zoology has, 

 however, shown that it is not possible to establish rigidly 

 defined primary divisions of the animal kingdom ; but that 

 any such divisions must inevitably be more or less artificial, as 

 including certain inosculating forms which lead by a more or 

 less insensible gradation into neighbouring groups. Thus, in 

 the group now in question, while there can be no doubt as to 

 the affinities which subsist between the Echinodermata and the 

 Scolecida, the latter, in turn, exhibit strong points of relation- 

 ship with the lower Annulosa. While, therefore, we must not 

 fail to recognise the points of resemblance between the Echino- 

 derms and the Scolecids, it seems, upon the whole, best to 

 separate the Echinodermata as a distinct primary division or 

 " sub-kingdom," and to regard the Scolecida as a special section 

 of the sub-kingdom Annulosa. 



The Echinodermata may be defined as follows : 

 Simple marine organisms, the body of the adult more or less 

 conspicuously radiate, that of the young often distinctly bilateral. 



