XVI 



ECHINODEKMATA 



463 



connected by a much less strongly ciliated band, which passes round 

 the dorsal side of the oesophagus just behind the mouth. 



It is commonly taken for granted that the function of the adoral 

 band is to direct a stream of water carrying minute organisms into 

 the mouth, and that it is in this way that the larva secures its 

 nourishment. Some years ago we made some observations on 

 the function of the homologous band in the larva of Echinus, 

 and it seems to us that the main function of the adoral ciliated 

 band, like the function of the cilia in the transverse grooves 

 running across the labial palps of Pelecypod Mollusca, is to remove 

 excess of food from the neighbourhood of the mouth. The minute 

 organisms, which constitute the bulk of the food, may be seen to be 

 carried in by a current which passes into the stomodaeum at its 

 dorsal border. This current seems to be caused by the cilia of the 



ctilong 



cillong 



oes 



FIG. 357.- -Young larvae of A sierras vulgoris. (After Field.) 



A, about three days old, from the side. B, about four days old, from the ventral surface. C, about 

 five days old, from the ventral side, a, anus ; cil.ad, adoral band of cilia ; cil.Jong, longitudinal ciliated 

 band ; int, intestine ; oea, oesophagus ; at, stomach ; stom, stomodaeum. 



principal longitudinal band (v. infra], aided no doubt by the cilia 

 of the dorsal side of the stomodaeum. At the ventral end of the 

 stomodaeum particles may lie seen to be flung outwards violently- 

 hence it is apparent that the current produced by the adoral band 

 is directed outwards. The food accumulated in the outer end of 

 the stomodaeum is transferred to the stomach, not by the action of 

 cilia but by peristaltic muscular contractions. 



Whilst these changes have been taking place other events have 

 been occurring. The cilia, which covered the whole surface of the 

 blastula and gastrula, become specially abundant and long over the 

 course of a sinuous band of thickened epithelium which is termed 

 the longitudinal ciliated band (cillowj, Fig. 357, A), and which is 

 the principal locomotor organ of the larva. Over the rest of the 

 surface they do not disappear, but become very sparse. This is due 

 to the passive stretching of the epithelial cells in these regions, 

 due to the increase in the pressure of the blastocoelic fluid. 



The longitudinal ciliated band is also found in the larvae of 



