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 (</) 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 , s 



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- 

 lacral ossicles of such Asteroids 

 as Astropecten. A transverse 

 section across an Ophiuroid arm 



is Shown in Fig. XXL (cf. p. 1 5). Diagrammatic section through oral region of 



Ophiuroid (after MacBride). amp, ampulla ; 1>, 

 ^. , . bursa ; i.m, interradial muscle of the disc ; 0.s, 



From SUch a type as Uplliura genital sinus ; g.r, genital rhachis ; m, mouth ; 



7 , i ., . i n , m.p, madreporic pore ; n.r, circumoesophageal 



CUtam, Wltll Its SmOOtn, nat, nerve ring . pmtt pe ristomal sinus ; st.c, stone 



circular disc, its irregular, scaly ^g^^fS^ 1 ^^'^ 

 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 



