210 



THE BIOLOGY OF MARINE ANIMALS 



Mouth 

 Pharynx 



Fig. 5.6. Feeding Currents 

 in an ectoproct polyzoan 



This is a longitudinal section 

 through the tentacular crown. 

 Arrows indicate direction of feed- 

 ing currents produced by action of 

 lateral cilia. (From Atkins, 1932.) 



form a funnel with the mouth at the base. A water current is produced by 

 the beating of lateral cilia on the tentacles, and proceeds straight down the 



lophophore and outwards between the ten- 

 tacles. Suspended food particles arriving 

 at the bottom of the funnel are sucked in 

 by the muscular pharynx, which is also 

 provided with inward -beating cilia. 



In endoprocts a rather different ciliary 

 mechanism prevails (Fig. 5.7). Water is 

 drawn into the tentacular funnel, again by 

 the action of long lateral cilia on the ten- 

 tacles, but the current proceeds from the 

 outside between the tentacles and upwards 

 away from the animal. Suspended particles 

 in the water passing between the tentacles 

 are thrown by the lateral cilia on to the 

 inner surface whence they are carried by 

 short frontal cilia towards the base. Here 

 they reach a ciliated vestibular groove 

 which leads to the mouth. The lateral cilia, 

 both in ectoprocts and endoprocts, beat 

 intermittently. In Loxosomella, for ex- 

 ample, all the lateral cilia may suddenly become motionless, and activity 

 is subsequently resumed in a somewhat irregular manner. It may be that 

 the lateral cilia are subject to nervous regulation (5a). 



The lophophore of Phoronis is similarly employed in feeding. The cilia 

 on the two sides of the tentacles beat in opposite directions, towards and 

 away from the mouth. An inhalant current passes downwards between the 

 two circles of tentacles forming the lophophore and outwards between the 

 tentacles. Particles are carried by tentacular cilia towards the mouth and, 

 if not ingested, they are borne outwards by distally-beating cilia to the 

 tips of the tentacles, where they drop off. Phoronids feed on fine plankton 

 and detritus. 



Brachiopods. These animals are provided with an internal ciliated 

 lophophore, bearing fringes of tentacles, which forms a complicated 

 filtering apparatus. In Crania, the lophophore divides the mantle cavity 

 into a lower inhalant and an upper exhalant chamber (Fig. 5.8). An 

 inhalant current enters on either side and is drawn upward through the 

 superposed whorls of the lophophore, which bear a double row of filtering 

 filaments. Currents are created by lateral cilia on the lophophore filaments, 

 body of the lophophore and mantle. Heavy particles drop down on the 

 lower mantle surface and are rejected by ciliary action, whereas the finer 

 particles become entangled in mucus on the filaments. These trapped food 

 particles are carried by frontal cilia to a buccal groove at the base of the 

 filaments, and are thence conveyed by a strong ciliary current to the 

 mouth. After passing through the lophophore the water currents become 



