274 THE STELLEROIDEA 



have no ambulacral groove. The arms are generally sharply marked off 

 from the disc, are generally five in number, and are sometimes elaborately 

 branched. The digestive system, which is aproctous, and the generative 

 system are both confined to the disc ; so also is the special respiratory 

 apparatus which takes the form of deep clefts. 



This diagnosis at once indicates that the Ophiuroids are more nearly 

 allied to the Asteroids than to other Echinoderms, for both classes are 

 actinogonidial, eleutherozoic, and lysactinic. Moreover, neither of the 

 main characters which separate the two classes hold in all cases ; for in 

 Ophioteresis and Protaster there is a ventral groove, and in some species of 

 Astroschema the arms pass gradually into the disc. Similarly the 

 Asteroids of the family Astropectinidae have no anus ; and the Ophiuroids, 

 Gorgonocephalus and Ophiactis, have no genital bursae. 



ORDER 1. Lysophiurae, Gregory (1897). 



Ophiuroidea in which the ambulacral ossicles are alternate, and are 

 not united into vertebral ossicles, but those of each segment are separate. 

 There are no ventral arm -plates, and the ventral side of the arm is 

 occupied by an ambulacral furrow. 



This order includes a group of fossil Ophiuroids, in which the arm 

 structure is asteroid in plan, for there are no ventral arm-plates; there is 

 an ambulacral groove ; and the ambulacral plates are in alternate pairs. 

 The members of the order differ from the Asteroids by having the arms 

 sharply marked off from the disc, and the alimentary canal was doubtless 

 confined entirely to the disc. 



FAMILY 1. PROTASTERIDAE. Lysophiurae with boot-shaped ambulacral 

 ossicles, each composed of a " body " in the median line of the arm, and a 

 lateral "wing" at right angles to it. Genera Protaster, Forbes, and 

 Bundenbachia, Stlirtz. There is a well -marked scale -covered disc and 

 five flexible arms. The axial portion or " body " of each ambulacral 

 ossicle is marked off into two parts by a depression which probably served 

 for the attachment of powerful ventral muscles. Stout adambulacral 

 plates occur in all the members of the family, and support lateral spines. 



FAMILY 2. PALAEOPHIURIDAE. Lysophiurae in which the ambulacral 

 ossicles consist of a bar -shaped or subquadrate "body" without wings. 

 Genera Sturtzura, Greg. ; Eugaster, Hall ; Ptilonaster, Hall ; Taeniura, 

 Greg. ; all Silurian ; and the Devonian Palaeophiura, Stiirtz. This family 

 is characterised by its alternate rod-shaped ambulacral ossicles, shown in 

 Palaeophiura lymani, Stiirtz. 



ORDER 2. Streptophiurae, Bell (1892). 



Ophiuroidea in which the ambulacral ossicles are opposite, and generally 

 united to form vertebral ossicles. In such cases the vertebral ossicles 

 articulate with one another by means of a more or less simple ball-and- 

 socket joint. The covering plates are more or less regularly developed as 

 superior, inferior, and two lateral, the last of which bear spines. 



