OPHIUROIDEA 635 



periodically shed ; in a number of forms asexual reproduction by division 

 of the disc occurs. 



Distribution and Habits. — The Ophiuroidea form the most numerous 

 class of echinoderms. They live on the sea bottom or among seaweed 

 and coral colonies, feeding on small animals or organic matter of all 

 sorts, and moving about by means of the long arms. 



History.— The brittle-stars were included by the earlier zoologists 

 among the starfishes, Linnaeus having placed them in the genus Asterias. 

 In 1816 Lamarck erected two new genera, Ophiura and Euryale, to include 

 all the brittle-stars, but it was not until 1840 that Mliller and Troschel 

 published the first extended classification of them. In 1841 Forbes first 

 made the group a separate class of the Echinodermata. The devel- 

 opment of the pluteus larv^a was first described by J. Muller. The modern 

 arrangement of the class is largely due to T. Lyman and F. J. Bell. 



The class contains more than 1,100 species, which may be grouped 

 in two orders. 



Key to the orders of Ophiuroidea: 



Oi Arms unbranched and mostly with distinct and regular plates, and 



usually without the power of roUing in towards the mouth. .1. Opiiiurae 



Oa Arms with superficial plates poorly defined or wanting, often branched, 



and with the power of rolling in towards the mouth 2. Euryalae 



Order 1. OPHIURAE. 



Brittle-stars with usually 5 unbranched arms which possess distinct 

 articulating axial plates and cannot (except some of the Ophiomyxi- 

 dae) be rolled towards the mouth: 7 families. 



Key to the families of Ophiurae here described: 



Oi Arms with distinct and regular superficial plates. 

 &i Lateral spines on the arms small and close to the surface. 



Cj Disc granulated more or less closely 1. Ophiodermatidae 



Ca Disc covered with scales or plates 2. Ophiolepidiae 



62 Lateral spines stand out from the surface of the arm. 

 Ci Oral papillae present. 

 di No cluster of tooth papillae at apex of each jaw. 



Ci Arm spines solid, often small and few:: 3. Amphiuridae 



63 Arm spines hollow, often long and numerous 4. Ophiacanthidae 



da A cluster of tooth papillae at apex of each jaw 5. Opiiiocomidae 



Ca No oral papillae present 6, Ophiotrichidae 



Ca Arms and disc without regular and distinct superficial plates, soft and 



flexible 7. Ophiomyxidae 



Family 1. OPHIODERMATIDAE. 



Oral papillae present, fringing the mouth; disc more or less closely 

 covered with granules; plates of arm regular and distinct, the lateral 

 plates bearing spines which lie close to the surface : 10 to 12 genera. 



