220 



THE BIOLOGY OF MARINE ANIMALS 



by the ciliated wheel organ and Hatschek's pit, where particles become 

 entangled in mucus and whence they are carried to the dorsal groove via 

 peripharyngeal ciliated bands (22). 



In balanoglossids the anterior end of the body consists of a proboscis 

 and collar which together form a simple ciliary mechanism for collecting 

 particulate matter and detritus. The proboscis secretes mucus which traps 

 particles and the resultant mucous strings are moved towards the mouth 

 under the impetus of cilia on the proboscis. A respiratory current created by 

 pharyngeal cilia assists in drawing in the mucous strings. Most entero- 

 pneusts are burrowers(e.g. Saccoglossus, Balanoghssus), inhabiting muddy 

 sand, and appear to ingest the surrounding soil for the sake of contained 

 organic matter without much selection. Some species are said to protrude 



Oil I bar 



Velum 



Ciliated 

 pit 



Wheel 

 organ 



Endostyle 



Fig. 5.16. Oral Hood and Anterior Region of Amphioxus, Showing 



Feeding Current (Indicated by Arrows). Particles of Detritus are 



Trapped on the Oral Cirri. (From Dennell (22).) 



their proboscis from the burrow on occasion, and gather up detritus from 

 the surface of the substratum. In any event, the gut of freshly collected 

 animals is swollen with sand and the casts consist largely of sand grains. 

 The pharynx is perforated by gill slits and the pharyngeal bars bear strongly 

 developed lateral cilia which create an outwardly directed respiratory and 

 feeding current. Particles are prevented from passing through the gill-slits 

 by the action of a sorting mechanism dependent upon latero-frontal 

 cilia, which carry such particles ventrally into the lumen of the pharynx. 

 Ciliary tracts lining the pharynx move particulate matter and mucous 

 strings posteriorly into the oesophagus, while surplus water is strained 

 off by the gill-slits and oesophageal pores (8, 54). 



In Cephalodiscus all parts of the body and stalk are covered with cilia 

 which transport food particles to the arms. These bear short cilia which 

 carry some particles distally to be discharged, and they are also provided 

 with a broad and shallow groove containing longer cilia which move food 



