THE BEXTHAL 



183 



projects out of the sand, and the water is conducted to the gill cavity 

 by variously arranged channels. This occurs in Calappa (Fig. 20) , 

 Matula, and Ilia. 5 In most crabs the water enters at the posterior bor- 

 der of the cephalothorax and is discharged forward. In the anomourus 



Fig. 18. — The heart urchin, Echinocardium cordatum, in the sand; the breath- 

 ing and feeding chimney is reinforced by a secretion ; the tubular arrangement 

 of the spines and one of the prehensile tube feet are shown. After v. Uexkiill. 



decapod Albunea (Fig. 21) the two large antennae together form a 

 long tube for the intake of the water for breathing, so that the crab 

 can bury itself in the sand without being cut off from its oxygen supply. 



Fig. 19. — Arenicola, from the right: g, gills; Id, dorsal setae; v, ventral setae; 

 e, excretory openings; Ih, head folds; p, papillae on the protruded pharynx. After 

 Ash worth. 



The number of forms concealed in the sand is astonishingly great. 

 Many sea anemones build tubes in the sand in which they can with- 

 draw completely. Turbellarian and nemertine worms conceal them- 

 selves in the sand from which they may be lured to the surface by 



