574 THE ANIMAL KINGDOM. 



plexus beneath the oesophagus. Irritation of the nerve causes only move- 

 ments in adjoining pinnules. There is an anti-ambulacral nervous system 

 composed of fibrils and ganglion cells, and both sensory and motor in 

 function, governing the flexion and extension of the arms in swimming. 

 It consists of a central mass lodged within or below (Comatulidae) the 

 circle of basals from which an extension passes down the stem with an 

 axial branch to every cirrus. Five axial branches enter the basals, 

 and each of them divides into two. Two adjoining branches enter each 

 radial and pass on, dividing as the arm divides and entering the pinnules. 

 A commissure connects all the ten secondary branches in the first radials, 

 and another commissure the two main branches in the axillary radials. 

 But there is some difference of detail in various Crinoids. The axial 

 nerves are at first lodged in a ventral groove throughout their course, a 

 condition permanent in some Palaeocrinoidea. But in all living Crinoidea 

 the calcareous matter finally surrounds them. The cords give off branches 

 in each joint, one pair to the thin aboral integument, two other pairs to the 

 lateral and oral surfaces. Bipolar cells have, in some instances, been seen 

 in the course of these branches. Their ultimate twigs appear to be con- 

 nected with the muscles (brachial) through stellate cells and through other 

 similar cells with the integument and with the tactile papillae furnished with 

 sensory hairs on the tentacles. A set of branches or a plexus at the side 

 of the ambulacral grooves in the disc and arms forms a parambulacral 

 system with a longitudinal connecting nerve in Actinometra nigra. And 

 in Antedon Eschrichti there is a circumoral labial plexus. 



The blood-vascular system consists of a circumoral ring with an 

 ambulacral vessel underlying the nerve ; of a dense circumoesophageal 

 labial plexus connected to the oral ring and sending out branches to 

 the genital plexus as well as intervisceral vessels ; of a plexiform organ 

 lying interradially in the disc anteriorly to the mouth and connected to 

 the labial plexus, genital and intervisceral vessels, and extending into the 

 ' chambered organ ' of the calyx. This organ lies between or below 

 (Cotnatididae) the basals, and within the central portion of the aboral 

 nervous system. It consists of five peripheral and radial chambers and 

 a central plexus or 12 vessels. Five vessels pass, one from each chamber, 

 to the corresponding cirrus and are continued down the stem dilating in 

 each nodal joint and sending off a branch to each cirrus. The plexus 

 supplies the remaining whorls of centro-dorsal cirri in Comattdidae, and is 

 continued down the stem in stalked forms. It has recently been shown 

 that two vessels separated by a septum lie in the centre of the axial 

 nerves of the arms. 



The water-vascular system consists of a circumoral ring and radial 

 vessels, all of which are beneath the corresponding blood-vessels. Cal- 

 careous subambulacral plates sometimes occur in the connective tissue 



