XII 



PHYLUM MOLLUSCA 



701 



the two valves. In Teredo (Fig. 583), the so-called Ship-worm, 

 which causes great 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. 584), which lives 

 buried in sand, there is a similar, but 

 wider calcareous tube, with which the 

 valves are completely fused, and the 

 anterior end of the tube which ap- 

 pears above the surface of the sand 

 is closed by a plate perforated with 

 numerous holes like the rose of a 

 watering-pot. The larval shell 



IS 



Fro. 582. Bcquienia ammonea ; B, Hip- 

 purites cornu-vaccinum. a, right 

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

 bridge. Natural History.) 



sometimes, though not always, dis- 

 tinguishable at the apex of each 

 valve in the Pelecypoda in general. 



In Nucula, Area, &c., the foot 

 (Fig. 586, ft.) presents what may be 

 considered 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. 577), 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. 585, F): in the Oyster it is absent. In addition 

 to the anterior and posterior retractors and a pair of protractors, 



FIG. 583. Teredo navalis, in 

 a piece of timber. P. pallets 

 (small calcareous plates support- 

 ing the siphons) ; . siphons ; 

 T. tube ; V. valve of shell. 

 (From the Cambridge Natural 

 Bistm-y.) 



