512 



THE INVERTEBRATA 



the mouth of the tube when the siphons are retracted. The foot is 

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

 cavity is maintained by ciUary 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. 



^•^- dUjl ct'. int. cm. 



Fig. 376. 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; inh.c. inhalant current; pcd. 

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



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 

 funnel or siphon^ 2i muscular organ, originally bilobed, used for the 

 expulsion of water from the mantle cavity ; one or two pairs of typical 

 ctenidia; coelom sometimes exceedingly well developed, the genital 

 part being continuous with the pericardium ; typically a chambered 

 shell in the last chamber of which the animal lives, though in most 

 modern representatives it is reduced and internal or wholly absent ; 

 nervous system greatly centralized and eyes of great size and often 

 complex type; eggs heavily yolked and development direct. 



