ECHINODERMATA. 3 



The Echinoderais are most highly organized animals, and for the most part are 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, developes spines of various forms and dimensions, which aid in locomotion, 

 and serve as defensive instruments to the creatures possessing them. By far the largest 

 number of these animals have a complicated system of vessels for the circulation of water 

 through their bodies. These aquiferous canals are intimately connected with the locomotion 

 of the animal ; for by means of it, most of the typical groups put in motion those remark- 

 able suckers which protrude in rows from different divisions of the body. In the Ecldnoidea 

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

 through apertures in the intervals of the small plates which form the middle of the rays, 

 whilst in the Sipuncidoidea 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 

 condition and other points of structure resemble the Polypi/era, others exhibit the 

 true rayed forms, clothed in prickly armour, which characterise the central groups of this 

 class. These conduct us, through a series of beautiful gradations, to soft elongated organisms, 

 whose forms mimic the Ascidian Mollusca ; whilst others have the long cylindrical body 

 and annulose condition of the skin, with the reptatory habits of the Ajjodous 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 

 Naturalists of our age — Tiedemann, Muller, Van Beneden, Agassiz, Desor, Forbes, and 

 Sharpey, — and have yielded fruits which the physiologist reckons as among the most 

 marvellous contributions to morphological science. 



The class Echinodermata is divided into eight orders, which, in descending sequence, 

 may be thus arranged : 



1. SlPUNCULOIDEA. 



2. HoLOTHUROIDEA. 



3. ECHINOIDEA. 



4. ASTEROIDEA. 



5. Ophiuroidea. 



6. Blastoidea. 



7. Cystoidea. 



8. Crinotdea. 



Order i. Sipunculoidea — form the apodal Annelidous Echinoderms ; they have a 

 long cylindrical body, divided into rings by transverse folds of the integument : they have 

 no tubular suckers, nor calcareous parts in their body, nor is it divided into a quinary 

 arrangement of longitudinal lobes : some have horny bristles, like the feet of many Annelida, 

 which they somewhat resemble : their mouth is sometimes surrounded by tentacula, which 

 are not, however, regulated by a definite number, nor disposed with the same regularity 

 as in the next Order. They are unknown in a fossil state. 



Type. Sipuncuhts edulis. Cuv. Pallas. 



