XII 



PHYLUM MOLLUSCA 



683 



destruction by boring into piles, ships' timbers, &c., the valves (V.) 

 remain very small and weak but movable, and the general surface 

 of the mantle secretes a continuous shelly tube which lines the 

 burrow. In Aspergillum (Fig. 594), which lives buried in sand, 

 there is a similar but wider calcareous 

 tube, with which the valves are com- 

 pletely fused, and the anterior end of 

 the tube which appears above the sur- 

 face of the sand is closed by a plate 

 perforated with numerous holes like 

 the rose of a watering-pot. The larval 

 shell is sometimes, though not always, 

 distinguishable at the apex of each 

 valve in the Pelecypoda in general. 

 In Niwula, Area, &c., the foot 



FIG. 592. A, Requienia ammonea ; B, Hip- 

 purites cornu-vaccinum. a, right 

 valve ; /, point of fixation. (From the Cam- 

 bridge Natural History.) 



(Fig. 596, /.) presents what may be con- 

 sidered as its most primitive form, 

 having a flat ventral surface or sole 

 upon which the animal creeps. Far 

 more common is the ploughshare-like 

 form we are already familiar with in 

 Anodonta and Unio, adapted for slowly 

 making its way through sand or mud. 

 In a few forms, e.g. Trigonia and Cardium 

 (Fig. 587), it is bent upon itself and is 

 capable of being suddenly straightened so 

 as to act as a leaping organ : in Mytilus it is cylindrical (Fig. 595, F) : 

 in the Oyster it is absent. In addition to the anterior and posterior 

 retractors and a pair of protractors, the foot is sometimes provided 

 with a levator muscle, particularly well developed in Nucula and 

 its allies. 



x x 2 



FIG. 593. Teredo navalis, in 

 a piece of timber. P, pallets 

 (small calcareous plates support- 

 ing the siphons) ; SS. siphons ; 

 T, tube ; V, valve of shell. 

 (From the Cambridge Natural 

 History.) 



