ECHINIDEA. 501 



Eckinodermata, 1 sea-urchins, or sea-eggs, derive their name 

 from these curious spinous processes. 



In most Echinidea all the feet are expanded into sucker 

 discs, at their extremities, and are here strengthened by a 

 calcareous plate or plates ; but in the Echino-cidaris and 

 some others, the feet of the oral portion of the ambulacra 

 only have this structure, while those of the apical portion 

 are pectinated, flattened, and gill-like. Miiller distinguishes 

 four kinds of feet in the Spatangoidea, — simple locomotive 

 feet, without any sucking disc ; locomotive feet, provided 

 with terminal suckers, and containing a skeleton ; tactile 

 feet, whose expanded extremity is papillose ; and gill-like 

 feet, triangular, flattened, with more or less pectinated 

 lamelhe 



In the Clypeastrodea the pefcaloid portions of the am- 

 bulacra possess branchial feet, interspersed with delicate 

 locomotive sucker feet, provided with a calcareous 

 skeleton. In the Ophiuridea and Crinoidea the feet are 

 tentaculiform ; and there are no vesicles at the bases of 

 the feet, while in the Asteridea they are well developed, but 

 simpler than those of the Echinidea. The madreporic canal 

 is, in the Asteridea, strengthened by a remarkable cal- 

 careous framework, which has given rise to the notion 

 that it is filled with sand, and to the name "sand-canal," 

 which has been applied to it. The canal terminates in 

 the madreporic tubercle, which is always placed inter- 

 radially on the antanibulacral surface of the star-fish. 



In some, Holothuridea, the feet are scattered over the 

 whole ambulacral region, as well in the inter- ambulacra 

 as in the ambulacra. In others, Psoitis, the feet are de- 

 veloped only from three of the five ambulacra ; while in 

 the Synuptoe and Chirodatcs there is only a circlet around 

 the mouth. 



Many star-fishes, and Synapta among the Holothuridea, 

 have the curious habit of breaking themselves up into 

 fragments when taken ; Miiller has pointed out the very 

 curious fact, that in Synapta, at any rate, this act may be 

 prevented by cutting through the oral nervous circle. The 

 nervous circle in the Echinus surrounds the oesophagus near 

 the mouth, and is enclosed by the alveoli, between which the 



(1) Derived from ecJii?ws, a st>ine. and derma, skin. 



