224 



CHAPTER XII. 



SUB-KINGDOM IILANNULOIDA. 

 ECHINODERMATA. 



SUB-KINGDOM III. ANNULOIDA. Animals in which the ali- 

 mentary canal is completely shut off from the general cavity of 

 the body. Nervous system distinct. A peculiar system of canals, 

 usually communicating with the exterior and containing water 

 derived from the outside, and termed the " water-vascular " or 

 " aquiferous " system, is present in all. In none is the body 

 of the adult composed of definite segments, or provided with 

 " bilaterally disposed successive pairs of appendages!' 



This sub-kingdom was proposed by Huxley, as a pro- 

 visional arrangement, to include the two groups of the Echino- 

 dermata (Sea-urchins, Star-fishes, &c.) and the Scolecida (Tape- 

 worms, Bound-worms, Wheel -animalcules, &c.) Whether 

 this arrangement be ultimately retained or not, matters not 

 at all to the palaeontologist, as no member of the Scolecida is 

 known in the fossil condition. The palaeontologist, therefore, 

 has simply to deal with the Echinodermata, the complete dis- 

 tinctness of which, as a group, is beyond question. 



CLASS ECHINODERMATA. 



The class Echinodermata comprises the animals known 

 commonly as Sea-urchins, Star-fishes, Brittle-stars, Sea-lilies, 

 and Sea-cucumbers, and is distinguished by the fact that the 

 external envelope of the body (" perisome ") has the power of 



