472 



COMPARATIVE A iX ATOMY 



The five i)airs of longitudinal muscles or pairs of muscle lamelUe in the Echino- 

 fhuridcc invite comparison with the live longitudinal muscles or pairs of muscles in 

 the Holothurioidea. No true homology can, however, be proved with certainty, 



Fig. 380.— Test of Asthenosoma, broken open so as to show tlie longitudinal muscles. 1, In- 

 terambulacral plates ; '2, anibulaeral plates ; 3, radial canals ; 4, centrum tendineum ; 5, muscle 

 bands ; (3, ambulacral apophysis (auricula) ; 7, opening muscle of the teeth ; 8, retractors of the 

 masticatory apparatus. 



since no direct relation between the calcareous ring of the Holothurian, and any 

 definite portions of the Echinoid skeleton (such as the auricule.s, or the pieces of 

 the masticatory apparatus) can be established. 



C. Asteroidea. 



On the apical side of the arms and of the disc, a dermomuscular 

 layer has been observed, which consists of external transverse, and 

 internal radial fibres, and runs lengthwise in the arm. This does not 

 appear to spread (as a muscle layer) to the oral side of the body, where 

 the ambulacral skeleton is developed. It may perhaps, however, have 

 become differentiated here into the special musculature of the ambu- 

 lacral skeleton. 



This latter is developed as follows : — 



Ten muscles occur in each skeletal segment. 



1. On each side two vertical muscles (or bands), one distal and the other 

 proximal, connect the adambulacral witli tlie ambulacral plate {cf. Fig. 309, p. 351). 



2. On each side an upper longitudinal muscle connects every two consecutive 

 ambulacral plates on their apical side (that turned to the body cavity). The func- 

 tion of these muscles is to bend the arm upward (Fig. 381, 2 and 7). 



3. On each side a lower longitudinal muscle connects every two consecutive 

 adambulacral pieces ; this muscle counteracts the upper longitudinal muscle. 



