OPHIUROIDEA 



149 



in a single genus (Ophioschiza) there is only one slit in each 

 interradius. 



In the bottom of the star-shaped mouth is a simple, round 

 opening that leads directly into the large, sac-shaped stomach, 

 which fills up the whole disc. There is no intestine and no anal 

 opening, the indigestible particles being expelled through the 

 mouth. No hepatic caeca. No part of the intestinal organs passes 

 into the arms. In the inner corner of each interradius a small, 

 stalked vesicle, the Polian vesicle, is found beneath the stomach. 

 The radial water - vessels are enclosed by the vertebrae. No 



ampullae to the tube-feet. The . 



radial nerve system well developed , 

 not in the epidermis, as in the 

 sea-stars, but in a furrow on the 

 ventral side of the vertebrae, 

 covered by the ventral plates. 

 Genital organs or gonads situated 

 at the base of the bursae ; in 

 some Asteroschematids they lie 

 partly in the base of the arms ; 

 they are mostly small, pear- 

 shaped bodies, serially arranged 

 (Fig. 85, 2), but in some forms 

 they have the shape of a larger, 

 leaf-shaped body. The sexual 

 products are emptied into the 

 bursae, from which they pass out 

 through the bursal slits. In the 

 rare cases where bursae are want- 

 ing, the sexual products are 

 emptied through an opening 

 formed in the body-wall for that 

 purpose. A few species are 

 hermaphrodites and at the same time viviparous ; this is the case 

 with four of the British species, viz. Amjyhipholis squamata, Am- 

 phiura borealis, Ophiomitrella davigera, and Oj^hiacantha anomala. 



The brittle-stars are generally rather lively animals, which 

 walk in a very peculiar way by means of the arms, these 

 being thrown forwards in pairs and then pushing the animal 

 forwards in jerks, almost as in leaps (Fig. 86). One of the arms, 

 any one of them, is directed in front or backwards and does 

 not actively assist in the walking. This way of moving is specially 

 characteristic of species living on a hard bottom {Ophiiira, etc.). 



Fig. 85. — 1. Diagrammatic trans- 

 verse section of the proximal 

 part of an arm, inside the edge 

 of the disk, showing the situa- 

 tion of the bursae and the genital 

 organs. 2. Diagrammatic fig- 

 ure of a bursa with the genital 

 organs attached to its wall (after 

 Ludwig). (From Danmark's 

 Fauna. ) 



b, Bursa ; d, Dorsal body - wall ; g, 

 Genital organ (gonad) ; gs, Genital or 

 bursal slit ; hv, Arm vertebra ; m, 

 Stomach ; v, Ventral body-wall. 



