224 THE BIOLOGY OF MARINE ANIMALS 



opening. The adoral bands of echinoderm larvae are represented in the 

 tornaria by ciliary patches above and below the mouth : the sub-oral appears 

 to be inhalant, driving particles inwards over the ventral lip; the supra-oral 

 is exhalant, driving surplus water outwards. Echinoderm and hemi- 

 chordate larvae are generally regarded as showing phyletic affinities (30). 



Tentacular Methods 



A few animals depend largely or entirely upon freely movable tentacles 

 for collecting fine food particles. The best examples are provided by certain 

 holothurians, such as Cucumaria, Thy one and Psolus. These animals, which 

 live in crevices or buried in mud, bear a crown of sticky tentacles about the 

 mouth. When extended they entangle plankton and other fine particles. 

 At intervals the tentacles are thrust one after another into the mouth 

 and adhering material is wiped off and ingested. There are also many 

 tubicolous and burrowing polychaetes (terebellids, spionids, etc.) possess- 

 ing extensile cephalic tentacles, which move over the surface of the bottom, 

 collecting particles and small organisms. This material is transferred to 

 the mouth by ciliary action (p. 207). 



Mucus-traps 



Secretion of mucus-sheets for entangling food particles is commonly 

 associated with ciliary feeding, as we have seen, and we have instanced one 

 form in which a mucus-bag is used as an alternative mode of feeding. This 

 animal, Chaetopterus, produces a mucous sieve, and draws water through 

 it by the pumping action of parapodial fans (p. 207). Very similar is the 

 feeding habit of the echiuroid worm Urechis caupo. This animal inhabits a 

 U-shaped burrow through which it pumps water by means of peristaltic 

 movements of the body wall (Fig. 14.6, p. 558). Near the anterior end of 

 the worm there is a ring of girdle-glands, which secrete a mucus-bag or 

 funnel. The mouth of this bag is fastened to the wall of the burrow, while 

 the lower end remains attached to the body wall. Water flowing along the 

 burrow passes through this net, which filters out particles down to 40 A in 

 diameter. Periodically, when the net becomes clogged, it is slipped over 

 the head, caught by the muscular proboscis and swallowed. 



Urechis feeds intermittently, periods of pumping alternating with 

 long periods of quiescence lasting from 20-60 min. During pumping periods 

 the animal is not always feeding, and a shift to feeding activity is accom- 

 panied by an increase in ventilation rate, from 1 1 to 29 c.c./min. Observa- 

 tions show that a worm produces a new feeding bag about once per hour, 

 and that the time spent in feeding amounts to 13 min in each hour (average 

 values). The metabolic rate of Urechis is known, but observations are not 

 at hand to relate these data (pumping rate and 2 consumption) to actual 

 food intake under normal environmental conditions. Feeding funnels, 

 produced by parapodial glands, are also used by Nereis diversicolor in 

 filter-feeding (36, 40, 63, 65). 



An interesting example of a mucus-trap is provided by a sessile gastropod 



