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 Huryaleae 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 



joes 
pasenes 
EO Ot 
LY 

On 
CNS een 
7 Q7 / 
Fic. 327. y ees 
Portion of central disk of Ophioglypha ? \ 
viewed from the ventral side. «, Mouth 
shield; 6, Side mouth shield; ec, Jaws Fic. 328 
covered by papillae; g, Bursal fissures ; the j 
h, Side arm-plates ; i, Pores for the emis- Portion of central disk of Ophioglypha viewed 
sion. of the tube feet, surrounded with from the dorsal side. a, Radial shields; 
tentacle scales ; k, Spines. 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 
oralia ; Mundecksticke), 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. HURYALEAER. Muller 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 Recent. 
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. 
