OPHIUROIDEA 165 



Oj)hiodermatid9e, rei)resented in the European seas by four 

 genera, is mentioned on p. 225. 



Key to the families of Ophiurce known from or likely 

 to he represented in the British seas. 



1. Disk and arms covered with a thick, naked skin,^ conceaUng 



the scales of the disk and the arm plates ; arm spines erect 



I. Ophiomyxidce 

 Disk and arms not covered with a thick skin ... 2 



2. Arm spines appressed, often rudimentary ... 3 

 Arm spines erect (or outstanding) ..... 5 



3. Disk on both sides closely covered by granules, which conceal 



the scales ...... OphiodermatidcB 



Disk not, or only partly, covered with grains ... 4 



4. Disk partly with granules, which leave, at least, the large, 



rounded radial shields naked. Arms long and slender, 

 flexible, fragile. Pores usually large, or wholly covered by 

 one large tentacle scale .... Ophioleucidce 



Disk scales distinct, not covered by granules, rarely (the genus 

 Ophiopleura) concealed by skin . . IX. Ophiolepidce 



5. Tooth papillae present ....... 6 



No tooth papillae, only a single vertical series of teeth . 7 



6. Mouth papillae present .... III. Ophiocomidce 

 No mouth papillae ..... II. Ophiotrichidce 



7. An unpaired (infradental) papilla on apex of jaw , . 8 

 A pair of (infradental) papillae on apex of jaw 



VI. Arnphiuridce 



8. Second pair of tentacle pores outside the mouth edge ; small, 



delicate forms, with smooth disk and very long, thin and slender 

 arms ...... VII. Amphilepidce 



Second pair of tentacle pores inside the mouth edge ; disk often 

 with spines or granules ...... 9 



9. Mouth papillae form a continuous series with the infradental 



papilla . . . . . . . . .10 



Mouth papillae do not form a continuous series with the infra- 

 dental papilla .... V. Ophiactidce (partly) 



^ A fairly thick skin is found also in some other forms, e.g. Ophiopleura, 

 Ophiotoma, and Ophiolebes. The first of these (of the family Ophiolepidtc) 

 is at once distinguished from the Ophiomyxidse by its rudimentary, 

 appressed spines. Ophiotoma (of the family Ophiacanthidae) has the arm 

 plates distinct, not covered by thick skin ; Ophiolebes, likewise of the 

 Ophiacanthidae, is distinguished {i.e. the two N.E. Atlantic species) from ' 

 the Ophiomyxidae by the disk being closely set with rather coarse 

 granules or globules. Also Ophiopsila (of the family Ophiocomidae) may 

 be mentioned here as having the disk apparently naked ; in reality it is 

 not so, but covered with exceedingly fine scales, and the plates of arms are 

 not concealed by skin ; this also holds good of Amphiura securigera, in 

 which the disk is really naked, wholly without scales, but with long, narrow 

 radial shields. 



