NUTRITION AND FEEDING MECHANISMS 207 



for capturing food particles. The greatest velocity, on the other hand, is 

 reached at the osculum, from which a jet of water is thrown upwards 

 away from the sponge. Waste materials, released by the sponge, are carried 

 upwards in this jet and then enter a slow circular eddy returning to the 

 sponge. The farther the exhalant current goes, the greater will be the 

 opportunity for waste materials to be dissipated and for water entering the 

 sponge to be renewed by diffusion, or water currents. The size of the osculum 

 determines the pumping rate and the velocity with which the water is 

 expelled. These are conflicting demands which counterbalance each other, 

 and for any particular sponge there is found to be an optimal oscular 

 diameter, which is proportional to the square root of volume of the 

 sponge. The leuconoid type of structure confers certain advantages, which 

 become most apparent in sponges of large size and those living in quiet 

 water. The increase in number of choanocytes brought about by folding 

 of the walls of the sponge results in greater current flow, and frictional 

 resistance is minimized by the smooth surface and absence of choanocytes 

 in afferent and efferent channels. Pumping rates in sponges and other 

 ciliary feeders are summarized in Table 4.5 (p. 169) (10, 48, 90). 



Coelenterates. The coelenteron in these animals is usually ciliated, and 

 in certain forms cilia are also employed in transporting food to the mouth. 

 In anemones the cilia may be widespread over the ectoderm (Protanthea) 

 or limited to disc, tentacles and stomodaeum (Metriclium). These ciliated 

 areas transport food particles to the stomach. Likewise in many corals the 

 general ectoderm participates in the capture of food, and ciliary tracts 

 transport particles to the mouth. Normally the cilia on the disc beat out- 

 wards, but in the presence of nutritive substances the direction of ciliary 

 beat is reversed and the mucus is drawn inwards. In the scyphomedusa 

 Amelia we also find that cilia on the bell and ventral surface of the arms 

 are concerned with transporting food particles to the mouth (91). 



Polychaetes. The tubicolous polychaetes for the most part are plankton 

 or detritus feeders and make use of ciliary mechanisms for obtaining food. 



Detritus feeders such as terebellids and ampharetids are provided with 

 long mobile tentacles which extend over the surface of the substratum. 

 Food particles (detritus, small organisms) are caught in mucus and trans- 

 ported along a ciliated groove in each tentacle towards the mouth (Fig. 

 4.2, p. 139). Small particles are moved by cilia, larger ones by muscular 

 action (terebellids). Flaps about the mouth in Cirratulus exercise a selective 

 function and sort out the edible particles. Other forms, such as Sternaspis 

 and Pectinaria, collect detritus beneath the surface (18<z). 



Filter-feeding is carried out by a variety of ciliary mechanisms in 

 chaetopterids, sabellariids, sabellids and serpulids. In Chaetopterus water 

 is propelled through the U-shaped tube by rhythmical beating of parapodial 

 fans (Figs. 5.3, 5.4). Copious quantities of mucus secreted by the aliform 

 notopodia entangle food particles suspended in the water current. Strings 

 of mucus, bearing food material, are carried in ciliated grooves on the 

 aliform notopodia to a median ciliated groove on the dorsal body surface, 



