PHYLUM ECHINODERMATA 



199 



FIG. 137. Pentaceros reticularis, oral aspect. A large starfish 

 common on the coast of Florida. (From Weysse.) 



from disc; viscera extend into arms; ambulacral groove on ven- 

 tral surface of arms; anus and madreporite dorsal. 



3. CLASS II. OPHIUROIDEA BRITTLE-STARS 



Distinctive Characteristics. Body flattened; arms distinct 

 from disc; no caeca nor gonads in arms; no ambulacral grooves 

 nor anus; madreporite on oral surface. 



Structural Peculiarities. The arms of the brittle-stars (Fig. 

 138) and basket- fish (Fig. 139) are noticeably different from 

 those of the starfish. They are slender and exceedingly flexible. 

 The ambulacral groove is absent, being covered over by skeletal 

 plates and converted into the epineural canal. Each arm is 

 covered by four rows of plates, one aboral, one oral, and two 

 lateral. Spines are restricted to the lateral plates. Within the 

 arm are plates which have fused together and are known as 

 vertebra. The muscular system of the arm is well developed. 



