652 



ZOOLOGY 



SECT. 



the two valves. In Teredo (Fig. 545), the so-called Ship-worm, 

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

 &c., the valves (r.) 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. 546) the valves 

 are completely fused to the tube, 

 the anterior end of which is closed 

 by a plate perforated with numerous 

 holes like the rose of a watering- 

 pot. 



In Nucula, Area, &c., the foot 

 (Fig. 548, ft.} presents what may be 

 considered as its most primitive 



FIG. 544. Requienia ammonea; B, Hip- 

 purites cornu-vaccinum. , right 

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



In'iilil' S'lli'l'ill llixtlll'll.) 



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, 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. 547, F): in the Oyster 

 it is absent. In addition to the anterior and posterior retractors 

 the foot is sometimes provided with a levator muscle (Fig. 548, /), 

 particularly well developed in Nucula and its allies. 



Immediately posterior to the foot a byssus-gland is fre- 

 quently found: it secretes a silky substance in the form of 

 threads, which serve to anchor the animal permanently or 



ss 



Fie. :.4.j. Teredo navalis, in 



a piece of timber. P. pallets; 

 x<. siphons ; T. tube ; V. valve 

 shell. (From the t'//<'//-<'';/'j 



