OPHIUROIDEA. 569 



radius except that which contains the madreporite. The vesicles may be 

 replaced by ramified caecal tubules (vasa ambulacralia cavi). The radial 

 vessels lie in a groove on the ventral surface of the ambulacral ossicles. The 

 branches to the tube-feet perforate the aboral end of the ambulacral ossicles. 

 The feet are simple and conical, and there are no ampullae. The two 

 first pair, or buccal feet, are derived from a branch of the ring itself which 

 bifurcates. Delicate calcareous plates are found in the walls of the ring 

 and feet. Globular cells tinged with haemoglobin occur in the water- 

 vascular system of Ophiactis virens. 



The mouth is hidden by the interradial oral structures. The digestive 

 tract is globular with radial expansions, which do not, however, extend 

 up the arms. It is lined by ciliated epithelium, and is fixed to the body 

 walls by connective tissue threads which cross the coelome. This cavity 

 is not large, and extends outwards above and below the ambulacral ossicles. 

 The genital organs consist of an infertile rhachis contained within the 

 aboral blood plexus, and connected to a numerous set of fertile genital 

 caeca, which are disposed in a vertical series, and open on either side of 

 a genital bursa close to its aperture. These bursae have delicate walls 

 of invaginated perisoma, which sometimes contain calcareous plates. 

 They extend inwards dorsally to the digestive sac, and open by slit-like 

 ventral apertures, one on either side the base of an arm. There are 

 therefore two bursae in each interradius. The bursal aperture is divided 

 in Ophioderma into an adoral and an aboral part ; the bursa itself is single, 

 but the genital caeca are confined to the dorsal portion, and to the 

 interradial wall. Currents of water flow in and out of the apertures, and 

 the bursae have probably a respiratory function. The adradial wall of 

 each bursa is supported by a genital plate. The genital caeca are sur- 

 rounded by a blood plexus derived on the interradial side from the aboral 

 ring, on the adradial from a special genital vessel. 



The free swimming larva is a Pluteus, and differs from the Echinoid 

 Pluteus in possessing a pair of lateral arms developed from the sides of 

 the anal portion of the ciliated ring. 



Some Ophiurida develope a phosphorescent light from the dorsal 

 aspect of the arms. Both sub-groups of the Class appear in the Silurian 

 strata. 



There are two sub-groups 



1 . Euryalida : devoid of plates in the integument ; with simple or branched 

 arms, which can be rolled up towards the ventral aspect at their tips ; pedicellariae 

 present. Astrvphyton, Trichaster, and Astronyx are the principal genera. 



2. Ophiurida : with simple arms, superambulacral shields, and five buccal 

 shields ; the disc has either a soft integument or is covered by granules or plates. 

 There are several families. 



See lit., pp. 194, 196. 



