THE BENTHAL 



185 



excrement (Figs. 17 and 23). These siphons are convergent adaptations 

 which have been developed in phyletically distinct groups of mollusks, 

 in adaptation to life in sand and mud. Siphons appear to be unknown 

 in mollusks before the Cretaceous. 



The lamellibranch genera, Venus and Tellina* among others, are 

 sand-inhabiting forms. The razor shell, Solen (Fig. 23), is notable for 

 its habit of digging tubes as much as 50 cm. deep, in which the animal 



Fig. 22. — Balanoglossus clavigerus in its tube in the sand (30-60 cm. in depth) ; 

 the height of the heaps of excrement is 3 to 6 cm. After Stiasny. 



retires from its surface position at any disturbance of the water. The 

 pismo clam of California burrows deeply into sand otherwise devoid of 

 large forms. Cardium, which does not bore deep in the sand, and which 

 can jump by means of its angular foot, has, in contrast, a shell strength- 

 ened by its rounded form and strong ribbing. 



A few echinoderms are sand dwellers. The irregular forms Brissopsis 

 and Echinocardium (Fig. 18) regularly live in sand, which they ingest 

 for their food supply as does the sand dollar Echinorachnius. The sea 

 cucumbers of the genus Synapta are also sand diggers and sand eaters. 

 Many brittle stars of the genus Amphiura live buried in the sand. The 

 brachiopods of the ancient genus Lingula (Fig. 24) are sand dwellers in 

 the eulittoral zone of the Indian and West Pacific oceans. They live in 



* Also Donax, Cytherea, Mactra, Tapes, Psammobia, and Scrobicularia. 



