MOLLUSCA 587 



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 and hemicellulose. 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 the mouth of the tube when the siphons are retracted. The foot 



/ 1 v T V 



^•^- di.(jh ct' int. cm 



Fig. 401. Teredo, represented boring in wood. The sawdust formed by the 

 rotatory movement of the shell valves, sh., is shown entering the mouth, M., 

 and the faecal pellets of undigested wood are shown as black masses in the 

 exhalant chamber, exh.c. Other letters : an. anus ; au. auricle ; ct. ctenidium ; 

 ct.' continuation of ctenidium as a ciliated ridge over the visceral mass; cm. 

 caecum of stomach filled with wood; c.s. position of crystalline style sac; 

 di.gl. digestive gland; F. foot; int. intestine; z«^.c. inhalant current; pcd. 

 pericardium ; pit. palette ; sh. left valve of shell ; ven. ventricle. Original. 



is very much reduced. A constant current into and out of the mantle 

 cavity is maintained by ciliary action, and the ctenidia, though so 

 greatly modified and elongated, constitute a collector mechanism; 

 but it does not seem that diatoms obtained in this way form any part 

 of the normal food of the creature, which exists almost entirely on the 

 carbohydrates furnished by wood which also contains small quantities 

 of proteins. 



Class CEPHALOPODA (SIPHONOPODA) 



Bilaterally symmetrical Mollusca with a radula and a well-developed 

 head which is surrounded by a crown of mobile and prehensile ten- 

 tacles, sometimes held to be part of the foot, which certainly forms the 



