

XIV] CLASSIFICATION 367 



fundamental structure of Annelida and Mollusca, etc. The only 

 plausible explanation of this is to be found in the hypothesis that 

 the young represent in a rough sort of way the ancestor from which 

 the Echinoderms were derived. 



When the Vertebrata are dealt with it will be pointed out that 

 the larvae of the most primitive forms of that group bear a striking- 

 resemblance to those of the Echinodermata, and that in the 

 embryos of many Vertebrata the coelom undergoes at first a similar 

 division to that which occurs in the Dipleurula, suggesting the con- 

 clusion that the highest groups in the animal kingdom are also 

 sprung from the same ancestor as gave rise to the Echinoderms. 



The phylum Echinodermata is classified as follows : 



Sub-phylum A. Pelmatozoa. 



Echinodermata which are fixed to some foreign object during the 

 whole or part of their existence by a jointed stalk springing from 

 the centre of the aboral surface. 



Class I. CRINOIDEA. 



Pelmatozoa with five long arms which repeatedly fork. The 

 genital organs are borne in the tips of the branches. 



Sub-phylum B. Eleutherozoa. 



Echinodermata which are free during the whole of their adult 

 existence and rarely fixed even during the larval condition. When 

 the immature form is fixed the stalk springs from the oral surface 

 near the mouth and is not jointed. 



ClaSS I. ASTEROIDEA. 



Eleutherozoa with arms (free radii) containing outgrowths of the 

 alimentary canal and open ambulacral grooves. The arms have 

 feebly developed muscles and locomotion is effected entirely by the 

 tube-feet. 



Ex. Asterias, Echinaster. 



Class II. OPHIUROIDEA. 



Eleutherozoa with arms sharply marked off from the central disc. 

 The arms do not contain outgrowths of the alimentary canal and 

 have closed ambulacral grooves. They have highly developed 

 muscles, and locomotion is entirely effected by the arms, the tube- 

 feet being purely tactile. 



Ex. OpUogtypha. 



