ECHTXODEKMATA. 1 09 



Orders V. and VI. Cystoidea and Blastoidea. 



These orders merely require to be mentioned here, as all the forms 

 included in them are extinct, and are unrepresented at the present 

 day by living species. In both, the body is enclosed in a kind of 

 box formed by jointed calcareous plates, and it was in most cases 

 permanently fixed to the sea-bottom by a jointed stalk or column. 

 The arms, which form so conspicuous a feature in the true Crinoidea, 

 were either absent or very rudimentary. Both orders are in many 

 respects closely allied to the Crinoidea, and they constitute prob- 

 ably the least highly developed sections of the whole class of the 

 Ech in odermata. 



Order VII. Holothuroidea. 



In this order are comprised the highest of the Echinodermata, all 

 very different in outward appearance from any of the forms we have 

 hitherto considered. They are commonly known as Sea-cucumbers 

 or Trepangs, but, except in warm seas, they are rare and inconspicu- 

 ous animals at the best. They are all more or less worm-shaped or 

 snail-like in foi-m, and they are not provided with any regular shell. 

 The skin is leathery, and generally contains calcareous matter in the 

 form of microscopic plates or spicules, or more rarely in the form of 

 larger overlapping plates. The body is capable of great changes of 

 form in the way of extension or contraction, being provided with 

 exceedingly powerful longitudinal and transverse muscles. Loco- 

 motion is effected by the alternate extension and contraction of the 

 vermiform body, or by rows of ambulacral tube-feet, like those of the 

 Sea-urchins, protruded through the integument. Typically, the tube- 

 feet are arranged in five longitudinal zones, as in the Sea-urchins 

 (fig. 68) ; but, in other cases, the tube-feet are irregularly distributed 

 over the whole body, and in a few cases they are emitted from the 

 lower surface of the body alone. In some forms {Synapta), there 

 are no tube-feet, and locomotion is effected by numerous minute 

 anchor-shaped spicules of lime, attached to plates sunk in the skin 

 (fig. 70). The water-vascular or ambulacral system is sometimes 

 quite rudimentary, but in other cases its internal arrangement much 

 resembles that of the Sea-urchins, except that the madreporiform 

 tubercle is not placed on the outside of the body, but hangs down 

 freely in the interior of the body. The circular oesophageal ring carries 

 generally one very long Polian vesicle (fig. 69, p) ; and the tube-feet 

 are provided with " ampullse." The mouth is situated at the anterior 

 end of the body, and is provided with a crown of branched tentacles 



