ix PHYLUM ECHINODERMATA 415 



The family Ecliinidce is one of about five families of the sab-order 

 Ectobranchiata, the members of which all differ from the other 

 sub-order Entobranchiata of the Rcgularia, or regular Sea- 

 urchins, in the possession of dermal branchiae, and in having the 

 auricles in the form of complete arches. 



The Sea-cucumber (Cucumaria or Colochirus) is a member of 

 the Stichopoda one of the families of the sub-order Dendrochiwtce 

 of the Pedata, or foot-bearing Holothurians. The Dcndrochirotce 

 differ from the Aspidochirotce the other sub-order mainly in 

 having arborescent instead of shield-shaped tentacles, and the 

 Stichopoda differ from the rest of the DendrocJiirotce in having the 

 tube-feet arranged in five regular zones. The genus Cucumaria 

 is distinguished from the rest by having ten tentacles with the two 

 ventral smaller than the others. Colochirus is closely allied to 

 Cucumaria, the principal distinction being the presence in the 

 former of papillae taking the place of tube-feet in certain 

 situations, as already noted. 



The Feather Star (Antedon rosacea) is a member of the family 

 Comatulidce, which is distinguished from the four other living 

 families comprised in the class Crinoidea of the Pelmatozoa, by 

 the absence of a stalk in the adult condition. 



6. GENERAL ORGANISATION. 



General Form and Symmetry. Like the Ccelenterata, the 

 Echinodermata are radially symmetrical, the body being capable 

 of division into a series of sub-equal antimeres along a series of 

 radiating planes at right angles to the principal axis. In the 

 majority of existing forms (Asteroidea, Ophiuroidea, and Crinoidea) 

 the radial symmetry is expressed in the external form of the body, 

 which is produced into a number of radially disposed parts, the arms 

 or rays, arranged around a smaller or larger central disc. But in the 

 Echinoidea the body is sub-spherical, and in the Holothuroidea 

 sub-cylindrical, the radiate arrangement being in these classes 

 indicated externally only by the distribution of the tube-feet, and 

 internally by that of certain of the systems of organs. 



Although, however, the general external form and the arrange- 

 ment of some of the internal organs in the Echinodermata indicates 

 a radial symmetry, it is invariably found that this radial arrange- 

 ment serves to hide a more primitive and more fundamental 

 bilateral symmetry. This is best marked in the larva, which has 

 pronounced bilateral instead of radial symmetry, but is quite 

 recognisable in the adult. In all Echinoderms there is, passing 

 through the primary axis, a plane the median plane along 

 which, and along which alone, the body is capable of being divided 

 into two equal or, to speak more correctly, approximately equal- 

 right and left halves. The existence of such a single median 



