CRINOIDEA. 283 



In a close suture, which is also called a synostosis, the plates are immov- 

 ably connected together. A syzygial joint or suture, or a syzygy as it is 

 sometimes called, is a close suture of two adjacent brachials, and is charac- 

 terized by the fact that the proximal plate of the pair, i.e. the one next 

 the calyx, does not bear a pinnule, while the distal one does. The proxi- 

 mal non-pinnuliferous piece of a syzygy is called the hypozygal, the distal 

 one the epizygal. Syzygial suture is also found in the stem : in this case 

 the lower piece of the pair or hypozygal is without cirri, while the upper 

 piece or epizygal bears cirri. In anchylosis the plates are cemented to- 

 gether and the line of separation is difficult to distinguish or absent. 



As in Ophiurids the ectodermal epithelium of the abactinal 

 side of the arms and calyx is not to be distinguished. The 

 epithelium of the ambulacral grooves is ciliated ; elsewhere it is 

 non-ciliated. The cutis contains the skeleton and its connective 

 tissue is very largely replaced by calcareous plates. 



There is no dermo-muscular system. The muscles are in 

 bundles connecting the movably-articulated skeletal plates. 



The central nervous system consists of a ventral ectoneural 

 portion, a deep oral portion and a dorsal apical system. 



The ventral ectoneural system (Fig. 197, 1) very closely re- 

 sembles that of Asterids and as in them consists of an epithelial 

 plexus, especially concentrated in the epithelium of the open 

 ambulacral grooves of both arms and pinnules and of the ecto- 

 derm immediately surrounding the mouth opening. 



The apical nervous system consists of a cap-like sheath of 

 nerve fibres and cells surrounding the chambered organ (p. 288) 

 and giving off interradially nerves, which bifurcate in the basals 

 (Antedon, Ehizocrinus) or amongst the radials (Bathycrinus) 

 into two strands, which. diverge and pass to join the correspond- 

 ing strands of neighbouring nerves (Fig. 196). The single cords 

 so formed are radial in position and called the radial nerves of 

 the apical system ; they run to the tips of the arms and of 

 their branches (Fig. 197, 8), and give off cords which similarly 

 traverse the pinnules (Fig. 198). In the Articulata the whole 

 system lies in canals the so-called axial canals in the skeletal 

 pieces of the calyx and arms, viz. the infra-basals, the basals, 

 the radials and the brachials. In some forms (Antedon, etc.) 

 there is in the primaxil (Fig. 196, E 3 ) a chiasma and a commissure 

 connecting the two nerves which result from the bifurcation of 

 the main nerve, and the same nerves are in all cases connected, 

 at the level of the radials, both with each other and with those 



