170 AN INTRODUCTION TO ZOOLOGY. 



Worms, there are forms that must be looked on as 

 much lower. 



The Eehinodermata are divided into the following 

 classes: the Ceinoidea, or Stone-lilies^ the Asteeoidea, 

 or Starfishes, the Echinoidea, or Sea-urchins, and the 

 HoLOTHUEOiDEA, Or Sea-cucumbers. 



The first are fixed forms, with a jointed stalk, bear- 

 ing the animal (forming the calyx, or cup) at the end, 

 like a flower. The mouth is at the middle of the 

 calyx, and there is an anus situated near it. The water- 

 vascular system branches, not into " feet," as in the 

 Starfishes, but into the divisions of the tentacles, called 

 pinnules, from their likeness to the small divisions of 

 a fern frond : they are very small, and very numerous. 

 Both the stalk and the cup, together with the branched 

 arms that surround the cup, are hardened by calcareous 

 deposits. Hence the animal is well preserved in the 

 fossil state, to which most of the species belong, this 

 being one of the groups which have passed their 

 maximum development, and are now on the decline. 

 The fossil forms abound in the limestones of the 

 Carboniferous and other Palaeozoic rocks ; the cup is 

 only found in particularly good specimens, but the 

 stalk joints are exceedinglj' common, forming pretty 

 objects, in which the central canal, that in the living 

 form contains bloodvessels, presents various symmetri- 

 cal shapes when seen in section. The living represent- 

 atives of the class are mostly deep-sea forms ; but one, 

 the Feather-star {Oomatula or Antctlon), is found on 

 the English coast. It is of a pink colour. The larva is 



