ECHINODERMATA. 15 



Classification op the ECHINODERMATA. 



The name Echinodermata was given by Klein, in 1734/ to the shells of Sea- 

 urchins called Echini. Bruguiere 2 subsequently gave the name Echinodermata to that 

 division of the animal kingdom which comprised the Star-fishes and the Sea-urchins. 

 Cuvier 3 included in his class Echinodermes, with Asterias and Echinus, the Holothuria, 

 animals destitute of the prickly skin of the more typical forms, and which had many 

 external affinities with some Mollusca ; and subsequently, in his ' Regne Animal,' 4 he 

 grouped in this class les Echinodermes sans pieds, forming the order Sipunculida, which 

 connect the Radiata with the Annulose Articulata. 



The Echinoderms are highly organized animals, for the most part covered with a 

 coriaceous integument. In several orders it is strengthened with numerous calcareous 

 pieces, which together form a complicated skeleton. The external surface of the skin, in 

 many families, develops spines of various forms, which serve as instruments of defence 

 or locomotion to the creatures possessing them. By far the largest number of these 

 animals have a complicated system of vessels for circulating water through their bodies. 

 These aquiferous canals are intimately connected with the life and motion of the animal ; 

 by means of this vascular water-system most of the typical groups erect those remarkable 

 suckers which protrude in rows from different divisions of the body ; in the Echinoidea 

 they escape through holes in the poriferous zones, and in the Asteroidca pass through 

 apertures between the small plates forming the middle of the rays ; whilst in the Sipun- 

 culida these organs are altogether absent. 



No class of the animal kingdom more clearly exhibits a gradation of structure than 

 the Echinodermata ; for, whilst some remain rooted to the sea-bottom, and in this sessile con- 

 dition resemble the Polypi/era, others, clothed in prickly armour, and exhibiting the true 

 rayed forms characteristic of the central groups, conduct, through a series of beautiful 

 gradations, to soft elongated organisms, whose outline mimics the Ascidian Mollusca, 

 whilst others exhibit the long cylindrical body, annulose condition of the skin, and 

 reptatory habits of the Apodous Annelida. 



With so fertile a field for investigation, it is not surprising that the minute anatomy 

 of the Echinodermata should have engaged the attention of some of the most distinguished 

 zoologists of our age, and have yielded fruits which the physiologist reckons as amongst 

 the most marvellous contributions to morphological science. 



1 ' Naturalis Dispositio Ecbinodermatum,' Jacobi Theodori Klein, 1734. 



2 'Tableau Encyclopedique des trois R£gnes de la Nature,' 1/91. 



3 'Tableau Elementaire de l'Histoire naturelle des Animaux,' 1798. 



4 ' Regne Animal destribue d'apres son Organisation,' 1834. 



