THE STELLEROIDEA 



267 



radial side of the bursae, and sends off a branch along the radial 

 side of each bursa. The parts of the sinus beside the bursae bear 

 the gonads. 



Fio. XIX. 



Gonads (g) and bursae (6) of Ophioglypha albida. (After Ludwig.) 



Arm Structure. As we have seen, there are no prolonga- 

 tions of the digestive or reproductive organs in the arms, 

 while branches of the nervous and water -vascular systems run 

 along the ventral side between 

 the vertebral ossicles and the 

 ventral arm-shields. It follows 

 therefore that the vertebral 

 ossicles are directly supra- 

 ambulacral, and are homologous 

 with the ambulacral ossicles, 

 and not with the supra-ambu- 



/v- 



m. p. 

 FIG. XX. 



Diagrammatic section through oral region of 



lacral ossicles of such Asteroids 

 as Astropecten. A transverse 

 section across an Ophiuroid arm 



is Shown in Fig. XXI. (cf . p. 1 5). D , agr ^ nmMlc section through oral region of 



Ophiuroid (after MacBride). amp, ampulla ; >>, 

 _ , . bursa ; i.m, interradial muscle of the disc ; </.-. 



rom SUCh a type as Uptimra genital sinus ; g.r, genital rhachis ; m, mouth ; 



T ,! .. ,1 a 4. m.p, madreporic pore; n.r, circumoesophageal 



CttianS, With its Smooth, Hat, ne ve ring . 3 ,. Si pe ristomal sinus ; st.c, stone 



circular disc, its irregular, scaly ^^^^'^ Jwueiau ' ttt8 ' *" 

 plates, its comparatively short, 



straight, unbranched, and regularly tapering arms, there are many 

 striking deviations, both in aspect and structure. 



The Exoskeleton may be either more or less complete than in 



