Phylum 3. EcMnoderma. 



The Echinoderma were formerly grouped with tlic^ Ccxjlontcni as 

 Radiata, on account of their both exhibiting a r a d i h 1 s y iii m c^ t r y 

 and in spite of their differing much in other respects; in the 

 Echinoderms, there is an early development of mcsoblast; a body- 

 cavity, a circulatory and a water-vascular system arc present. 



3 



4, 



Pig. 75. Diagrammatic fignrea explaining the radial structure of the Echinoderma. 

 1 Star-fish from beneath. 2 Sea-urchin from beneath. 8 Sea-urchin, lateral view. 

 4 Holothurian from the side, a anus, o mouth, r radius, i inter-radius, I lines indicating 

 the scissions by which the animal may be divided into rays, t tentacle. — Orig. 



The characteristics of the fundamental form of a I'Ogular 

 Echinoderm are as follows : the body is usually pentamerous, i.e., it 

 may be divided into five approximately identical rays (antinierc^s) 

 by five scissions meeting in the principal or median axis. 

 The external form of the body varies in accordance with the l(uif.^-th 

 of this axis ; when it is longer than the transverse, the body is 

 elongate; when the axes are equal, or the transverse is slightly loiij^-er, 

 it is almost spherical; if the principal axis is much shorter, the body 

 becomes discoid. All these various types are connected by transitional 

 forms. The mouth lies at one pole of the principal axis, the oral 

 pole. The surface of the body may be divided by meridians into 

 ten segments, five of which, termed radii, bear tlie tube-feet 



