Chap. III.] ECHINODERMS. 63 



which exactly divides the body into two similar 

 halves. 



The Echinoderniata are sharply divisible into 

 two grades ; in the lower of these the animal is either 

 fixed by a stalk throughout life, or, as in the case of 

 the Rosy Feather star (Antedon rosacea) of our own 

 shores, the larva is fixed by a stalk (Fig. 24). This 

 grade may be called that of the Pelmatozoa ; to 

 it belongs the order of the Crinoidea, with others now 

 extinct ; representatives of it are Rhizocrinus, Pen- 

 tacrinus, and Antedon. 



In the organisation of these forms attention should 

 be directed to the presence of the cuplike central 

 portion ; this calyx consists essentially of a central 

 plate and two sets of alternating plates five in num- 

 ber ; these are the foasals and the radials. 



In the higher grade of the Echinodermata, the 

 Echinozoa, these plates are often obscured. In the 

 regular Sea-Urchins (Echinoidea) the two sets of 

 five plates can always be made out, but the central 

 plate is excavated to make room for the anus ; five 

 of the plates become perforated by the genital ducts 

 (basals), while the other five (radials) are similarly 

 perforated by the ocular tentacles. Cidaris, Echinus, 

 Echinometra are examples of the regular Echinoidea ; 

 by Clypeaster and the flattened Laganum we pass to 

 the edentulous Spatangidse, where a secondary bi- 

 lateral symmetry becomes very apparent. 



In the true starfishes (Asteroidea), of which 

 Asterias, Linckia, Oreaster, and Astropecten are 

 examples, and in the Ophiuroidea, of which Ophiura, 

 Ophiocoma, and Ophiothrix are representatives, the 

 calycinal plates are often obscured, and the ambulacral 

 suckers are limited to the lower surface of the body 

 and do not extend, as in Echinus, from mouth to 

 apex ; in the latter the ambulacra are covered in by 

 a ventral plate, and in one division (that of the 



