ANTEDON -ANATOMY 585 



plates of the patina. These coixls rapidly i'oik, and diu' di\ision 

 of each of two adjacent curds enters the lowest radial. In the 

 third radial all the cords are connected l)y a connnissure wliich 

 runs completely round the calyx. Eacli of the cords in the tliird 

 radial forks again, and one lirancli of eaeli cord enters eaeli of 

 the two arms connected with it, and the two branches entering 

 an arm coalesce to form a single cord. In the arms, as in the 

 calyx, the cords are deeply embedded in the ossicles, but branches 

 extend to the ventral surface of the arms and here unite to form 

 two longitudinal cords, one on each side of the groove. In the 

 tegmen these cords are connected by an outer nerve -ring, 

 branches from whieli join tlie ectodermal nerve-ring already 

 described. 



The researches first of W. Ix Carpenter^ and then of Marshall - 

 have proved that it is the aboral nervous system which really 

 controls the movements of the animal. If the chambered organ 

 is destroyed by cautery, the whole movements of the animal are 

 paralysed ; but it will carry out its characteristic swimming move- 

 ments just as well if the whole tegmen with the ambulacral 

 nerve-ring and. the whole of the alimentary cjinal are torn away. 

 The commissure in the third radials co-ordinates the movements 

 of the arms. If it is cut they move independently of one another. 

 The position of the radial cords inside the ossicles is gradually 

 acquired. At first they are gutter-like evaginations of the coeloni ; 

 by upgrowth of their sides the gutters become canals, and are 

 then surrounded by calcified tissue. The cirri have each a cord 

 traversing them which originates from the chambered organ. 



Goelom. — In the young stalked form the coelom consists of 

 the water -vascular system (" hydrocoel "), and underlying it 

 an oral coelom, separated from an aljoral coeloni by a horizontal 

 mesentery. As the animal grows, tliis horizontal mesentery 

 becomes largely absorbed, and the coelom becomes everywhere 

 traversed by cellular cords (trabeculae), which are later calcified. 



Both oral and aboral coelom become, like the hydrocoel, bent 

 into hoops, and along the axis of the aboral coelom a cord of 

 germ-cells is developed, which constitutes the " genital stolon." 

 The chambered organ is developed from the aboral coelom, and 



^ "Structure, Physiology, and Devulopinent oi Jntcdoii 7-usaecus," Pliil. Trans. 

 Hoy. Snc. 1866, pp. 671-756. 



- "On thu Nervous System o{ Anted on rosacea!'," Q.J. M.S. ,\.\iv. 1884, p. .^.07. 



