INTRODUCTION, 



17 



leaving it ropresonted by a sinus, the epinoural canal. The ventral shields are 

 formed after closure of the groove. The dorsal shields correspond to the median 

 ossicles on the apical surface of the arm of the Asteroids. 



We may also notice at this stage the following comparisons between the 

 musculature of the Asteroid and Ophiuroid arms. 



PROXIMAL. 

 Ch. 2 



] M:,. 



— cup. 



r.t.f. 



Text-fig. 14. — Oral view of vertebrae of Ophiarachna incrassata (after LiicUvig-). Ch., ambulacral channel; 

 M^., adoral excavation for the ventral longitudinal muscle; cup, cup for the tube-foot; f. t. .f., canal 

 from water- vascular system to tube-foot ; 1, point of entry of water-vascular branch to tube-foot ; 2, 

 the same of nerve to dorsal longitudinal muscle. 



A. The longitudinal (writhing) muscles are of course much more important in 

 Ophiuroids than in the Asteroids. 



(a) The dorsal longitudinal muscles present in both Asteroids and Ophiuroids 

 are much larger in the latter than in the former group. 



rROXiMAr,. 



Text-fig. 15. — Side view of vertebree of Ophiarachna incrassata (after Ludwig). r., ridge for articulation of 

 side shields ; can., branch vessel to tube-foot, which here takes a convoluted course through the ossicle. 



(b) The ventral longitudinal muscles are attached to the adambulacralia in the 

 Asteroidea. Homologous muscles are absent in the Ophiuroidea, where they are 

 replaced by muscles between the ambulacralia (vertebrae) — a much more advan- 

 tageous position. 



B. The cross-muscles between the opposite ambulacralia are only present, at 

 any rate functionally, in the Asteroidea. They would be useless in the Ophiuroidea 

 in consequence of the fusion of the vertebral halves. 



All these changes can be followed in fossil forms. 



(ii) The Structure of the Vertebras in Detail. — It is best to concentrate attention 

 upon the following points : 



3 



