586 THE INVERTEBRATA 



in some echinoderms. Another point of physiological importance is 

 the great part which leucocytes play in digestion ; the lumen of the 

 alimentary canal is invaded and diatoms and similar bodies ingested, 

 digested and transported by the leucocytes into the connective tissue. 

 A figure of the veliger larva of Ostrea is given (Fig. 375 A) to show 

 the ciliary currents by which food is obtained, the crystalline style, 

 which is revolved by the action of the cilia of the style sac, and the 

 foot, which is lost in the adult. 



ex.c 



ven.' 



^int. 



d.bx.' 



Fig. 400. 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. 



Teredo (Fig. 401) is the most specialized of the boring lamelli- 

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

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

 and Saxicava in the hardest limestone. Teredo and Xylophaga bore in 

 wood. The latter makes shallow pits, but Teredo, working with ex- 

 traordinary 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 rhythmical contraction. The sawdust is 

 swallowed by the animal and is largely retained in a relatively 



