in. 3 PTEROBRANCHS AND POLYZOANS 59 



Like the Pterobranchia the Polyzoa Ectoprocta are sessile, with mouth 

 and anus pointing upwards. They feed by means of the cilia borne on 

 a horseshoe-ring of tentacles (the lophophore); but there is no division 



Fig. 31. Longitudinal median section of Cephalodiscus. 



a. anus; b.c. 1, 2, and 3 body cavities; int. intestine; lo. lophophore; 



m. mouth; nch. 'notochord'; n.s. nervous system; oes. oesophagus; 



op. operculum (collar); ov. ovary; ph. pharynx; pp. proboscis pore; 



ps. proboscis; St. stomach; st.k. stalk. 



(Modified after Harmer, Cambridge Natural History, Macmillan.) 



into proboscis, collar, and trunk, and no tripartite coelom. The nervous 

 system is in the condition of a sub-epithelial plexus, which is folded, 

 around the base of the lophophore, to form a hollow tube — a remark- 

 able point of similarity to the chordates. Even though it is difficult to 

 compare this tube exactly with the nerve-cord of chordates, it is at 

 least evidence of the organization of the nervous system on a plan that 

 allows of such folding. It is probable that the modern pterobranchs 

 are the surviving members of the ancient group of graptolites, but 



