CHAPTER IV 



THE EGHINODERMA: STARFISHES, SEA URCHINS. AND 

 HOLOTHURIANS 



ALL the creatures included in the great division of the Animal 

 Kingdom described in this chapter are inhabitants of the sea, 1 

 and owe their name of the Echinoderma 2 to the prickly nature 

 of their skin, which is more or less beset with spines, granules, 

 or plates of carbonate of lime. The division includes the Star- 

 fishes, Sand and Brittle Stars, Feather Stars, Sea Urchins, and 

 Sea Cucumbers or Holothurians : all creatures characterised by 

 a prevailing radial symmetry of both their external and internal 

 structure ; while the presence of a gut distinct from the body- 

 cavity (ccelom) shows an advanced superiority of organisation 

 sufficient to separate them clearly from the Ccelenterates, with 

 which they were associated by the early zoologists. 



The Echinoderma are a most deeply interesting group of ani- 

 mals, not only on account of their great antiquity and beauty of 

 structure they range in time from the Cambrian through all 

 succeeding geological formations but also because of the won- 

 derful series of changes or transformations through which they 

 pass before reaching the adult stage ; changes of form so profound 

 that no one who had not followed the transformation step by step 

 would, on being shown a larva and adult, recognise them as being 

 the same animal at different stages of its life. The sexes are dis- 

 tinct in most of the Echinoderma, and the fertilised ova hatch 

 as uniformly ciliated free-swimming embryos, which gradually 

 develop a digestive tube with two openings. The cilia become 

 restricted to one or more transverse ridges, and the larva passes 

 from a more or less spherical condition into one exhibiting 

 a very complete bilateral symmetry, and thence by a series 



1 A Holothurian (Synapta stmilis) is said by Ludwig to enter the brackish waters 

 oi the tropical mangrove swamps. 



2 Greek, echinos, hedgehog ; derma, skin. 



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