PHYLUM MOLLUSCA 



701 



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

 which causes great destrdction by boring into piles, ships'-timbers, 

 &c., the valves {v.) remain very small and weak biit movable, and 

 the general surface of the mantle secretes a continuous shollj' 

 tube which lines the burrow. In 

 AspergilluTn (Fig. 584), which lives 

 buried in sand, there is a similar, bub 

 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 



Picj. 582.— Bequienia ammonea ; B, Kip- 

 purites comu-vaccinum. a, right 

 valve : /, point of fixation. (From the Cmn- 

 blHdf/e Natural History,) 



sometimes, though not always, dis- 

 tinguishable at the apex of each 

 valve in the Pelecypoda in general. 



In Ntocula, 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 inud. 

 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, 



ss'-X 



Fif!. 5S3.— Teredo navaliS; in 



a piece of timber. P. pallets 

 (small calcareous plates support- 

 ing the siphons) ; ss. siphons ; 

 T. tube ; \'. valve of shell. 

 (From the Cuiuhridge Natural 

 IlistoYy.) 



