xx ANTEDON ANATOMY 587 



The muscles of Antedon are of two kinds. Those of the 

 water-vascular system are, as in Eleutherozoa, basal outgrowths 

 of the cells forming the walls of the system. The muscles 

 moving the joints of the arms appear to be modifications of 

 connective-tissue cells. When the brachials are isolated their 

 terminal faces, strikingly long, recall those of Ophiuroid vertebrae. 

 There is a ventral groove for the coelomic canal. Above this 

 groove the face is divided by ridges into four areas for attach- 

 ment of the muscles. Dorsal to this is the pit for the strong 

 ligament which binds the ossicles together ; then comes the canal 

 for the aboral nerve-cord, whilst dorsal to this is the pit for what 

 is called the "dorsal elastic ligament." The theory underlying 

 this name is that the muscles bend the arms ventrally, and the 

 ligament by its elasticity restores them to their places ; but there 

 seems reason to believe that the " ligament " is really a dorsal 

 muscle. It is particularly to be noted that similar muscles occur 

 between the first and second radials, proving that the primary 

 arm really begins with the first radial. The second and third 

 radials, as also the first two ossicles and certain others of each 

 arm, are closely united by calcified fibres, and this kind of union 

 is called a "syzygy" (Fig. 265, syz). The cirri have all their 

 ossicles united by muscular attachment, and can move rapidly. 



The blood system (see pp. 449-451) forms a ring consisting 

 of a network of strings round the oesophagus. This is termed 

 the " labial plexus." From this cords can be traced to the wall 

 of the stomach and to the surface of the genital stolon. The 

 assertion that radial strands intervene between the ectodermic 

 nerve-cord and the radial water- vascular canal, though usually 

 made, does not appear to be justified, since what is termed the vessel 

 appears to be a crevice formed by shrinkage in preservation. 



The process of respiration is doubtless largely carried out by 

 the podia, but it must be assisted by the constant instreaming of 

 fresh sea-water through the pore-canals. The process of excretion 

 has not been directly observed in Antedon, but structures called 

 " sacculi " may be connected with this function. These are 

 spherical masses of amoebocytes embedded in the tegmen. 

 During life they are colourless, but after death they become 

 coloured, showing that they secrete a peculiar compound. These 

 sacculi abound in the disc, and a row of them is to be found at 

 each side of the ambulacral groove in the arms. When, as in 



