XX ANTEDON ANATOMY 583 



a study of the development, and it can further Ije shown that 

 the arms fork repeatedly ; but in these further bifurcations one 

 fork remains short, and forms a pinnule, whilst the other con- 

 tinues the arm. Thus the arm, instead of being a single axis, 

 is really a series of axes — in a word, it is a " sympodium." 



If in the case of any bifurcation the two forks were to 

 develop equally, the number of arms in that ray would be 

 doubled, and this actually happens in the case of other species 

 of Antedon. 



Digestive System. — The mouth leads through a short vertical 

 oesopliagus into an enlarged stomach, which lies horizontally 

 curved around the axis of the calyx. The stomach is succeeded 

 by a short intestine, which leads into the anal papilla. Both 

 oesophagus and stomach are ciliated, and the food consists of 

 minute organisms, swept into the mouth by the current produced 

 by the cilia covering the ambulacral grooves and podia ; the ten 

 arms may indeed be compared to a net spread out in the water 

 to catch swimming prey. 



The water- vascular system consists of a ring closely surround- 

 ing the mouth, from which radial canals are given off which 

 underlie the ambulacral grooves and bifurcate with them. 

 The podia have no ampullae, but muscular strands traverse 

 the cavities of the radial canals, and that of the ring-canal, 

 and by their action water can be forced into the podia, 

 which are thus extended. Numerous stone-canals hang down 

 from the ring-canal, and open freely into the coelom ; they 

 do not, as in Holothuroidea (where the same arrangement 

 occurs), end in sieve-like madreporites. The tegmen, i.e. the 

 ventral surface of the calyx, is pierced by a number of 

 isolated pores lined by ciliated cells, which suck in water. In 

 the oldest Pelmatozoa there seems to have been a regular madre- 

 porite. In the larva of Antedon there is but one pore -canal, 

 which, as in most Eleutherozoa, leads into a special section of 

 the coelom, the " axial sinus," embedded in the body- wall, with 

 which also the single stone-canal communicates ; but later the 

 division between the axial sinus and the rest of the coelom breaks 

 down, and then the pore-canals and stone-canals become multiplied 

 hidependently of each other (Fig. 266, m.p, 2).c, and st.c). 



Nervous System. — In the young stalked form the nervous 

 system, as in other Echinoderms, consists of a ring round the 



