202 



ECHINODERMATA ASTEROZOA 



SUB-BRANCH II 



(Ophiura), appearing as two slits one behind the other; and in the fossil 

 Euryaleae they are often represented by rows of pores. 



The integument covering the entire upper surface of the disk and the 

 interbrachial area on the ventral side is frequently beset with calcareous 

 plates ; but this scale coat may be covered in turn with a thick skin, or bear 

 spines or granules. A large central plate is sometimes recognisable on the 

 dorsal aspect of the disk, together with five pairs of plates, which, from their 

 position at the points of origin of the arms, are called radial shields (Scutella 

 radialia, Fig. 328). On the ventral surface of the disk, the inner angle of 

 each interbrachial space is occupied by a single large plate termed the mouth 

 shield (Scutum buccale), one of which serves as the madreporic body. But in 

 the Euryaleae the mouth shields are feebly developed, or may be wanting 

 altogether ; and in place of them a madreporite is found in one or all of the 

 interrays. The mouth shields are bounded proximally by a pair of somewhat 



FIG. 327. 



Portion of central disk of Ophioglypha 

 viewed from the ventral side, a, Mouth 

 shield ; b, Side mouth shield ; c, Jaws 

 covered by papillae ; g, Bursal fissures ; 

 h, Side arm-plates ; i, Pores for the emis- 

 sion of the tube feet, surrounded with 

 tentacle scales ; k, Spines. 



FIG. 328. 



Portion of central disk of Ophioglypha viewed 

 from the dorsal side. a, Radial shields ; 

 b, Upper arm-plates; c, Side-plates. 



smaller plates called the side mouth shields (Scutella adoralia). Finally, within 

 the side mouth shields, and usually pressing against them, are the jaws (Scutella 

 or alia ; Mundeckstiicke}, which are sometimes covered by the skin or by granu- 

 lations (Fig. 327). Teeth are constantly present, being attached to the jaw- 

 plates by small muscles. 



The Ophiuroidea are divided into two orders : Euryaleae and Ophiureae, both 

 of which have fossil representatives dating as far back as the early Palaeozoic 

 era. For a precise systematic determination it is necessary to know the 

 character of the bursal fissures' and the mouth frame ; but such evidence is 

 not always obtainable in the case of fossil specimens, owing to imperfect con- 

 ditions of preservation. 



Order 1. BUR YALE AE. Miiller and Troschel. 



Arms dichotomously dividing soon after their origin (exceptionally simple 

 throughout their entire length), capable of being rolled up inwards, and covered with 

 a granulated or finely scaled integument. Mouth shields rudimentary or absent. 

 Genital fissures often represented by rows of pores. A madreporite present in one or 

 all of the interrays. Silurian to Eecent. 



The Palaeozoic species differ from all of the recent Astrophytidae in that 

 radial shields are not present, and that the arms bear a row of simple marginal 

 spines on the ventral aspect. 



