PRESIDENTIAL ADDRESS. 



39 



save itself from hurt, the Donax has assumed a thick shell 

 with a wedge shape and a slippery surface. The animal is 

 extremely strong and active, when outstretched, the foot 

 is as large as, and the siphon longer than, the valve. A few 

 drives of the powerful flat pointed foot draws the creature 

 under the sand. Here it stands upright, with the extended 

 siphons held aloft, their tips alone projecting above the 

 ground. The orifice of the inhalent siphon expands in a 

 disk beset with minute branched papillae. Of these there 

 are six main rays with secondary and tertiary intermediates 

 on the margin. Perhaps the papillae grasp small but 

 struggling animals. The exhalent siphon has no disk, its 

 margin is fringed with papillae. A parasitic crab, perhaps 

 an undescribed species of Pinnotheres, infests the Donax. 

 The mollusc is caught and eaten by the Red-bill, Koematopus 

 longirostris. This bird breaks off the anterior end of the 





h \ 



Fig. 6. Hermit crab of the sand beach, Diogenes custos, magnified. 



