LAMELLIBRANCHIATA 



511 



Teredo (Fig. 376) is the most specialized of the boring lamelli- 

 branchs. While most lamellibranchs burrow in mud, others tend to 

 work in consolidated sediments, Pholas in chalk and sandstone, and 

 Saxicava in the hardest limestone. But Teredo and Xylophaga only 

 bore in wood. The latter makes shallow pits, but Teredo, working with 

 extraordinary speed, excavates long cylindrical tunnels (sometimes as 

 much as a foot in a month or two). The wood is reduced to sawdust 

 by the rotatory action of the two shell valves, in which the adductor 

 muscle fibres maintain a constant rhythmical contraction. The saw- 



ea:.c. 



-int. 



d.h.c- 



Fig- 375- Ostrea edulis, general anatomy, right valve and mantle removed. 

 After Yonge. Lettering as in Fig. 374; in addition: in.c. inhalant and ex.c. 

 exhalant chamber; d.b.c. division between above chambers. Arrows indicate 

 direction of currents. 



dust is swallowed by the animal and is largely retained in a relatively 

 enormous caecum of the stomach, but a great deal of the material 

 passes into the cavity of the digestive gland and is there ingested by the 

 epithelial cells. There is no doubt that Teredo has developed enzymes 

 which are almost unique in the Animal Kingdom, which digest 

 cellulose, hemicellulose and probably lignin. The structure of the 

 animal is remarkable for the extraordinarily long siphons and mantle 

 cavity; while the mantle often lays down a calcareous lining to the 

 tube and always a pair of calcareous valves, the pallets, which close 



