314 



MOLLUSCA 



Even more room in the \-isceral sac is (Icmanclcd by the digestive tract in 

 wliich ccsophagus, stomach, a coiled intestine, a \ohiniinous li\er, ami 

 usually salivary glands may he recognized. Tiie liver is usually a paireii 

 tubular gland, emptying into the stomach. It not only digests fat and 

 stores up glycogen, but forms an enzyme (I'vlasc) which con\erts cellulose 

 into sugar. The radula or lingual ribbon is also a characteristic organ, 

 and its absence from the Acephala is probably tire result of degeneration. 

 It is a plate or Ijand armed with teeth wliich lies on the lloor of the pliarynx 

 on a ventral ridge, tlie tongue, and is used for the comminution of food 



(figs- 334, 335)- 



Reproduction is exclusivelv sexual; budding, fission, or partheno- 

 genesis being unknown. The eggs are usually united in large numbers, 



fSp J 



U!ir 



Ml 3RJ. 



Fig. 313.— \'cliger larva (irochophorc) of Teredo navalis (from Ilatschck). .-1, 

 anus; J, stomach; 7,, intestine;/,, liver; L.l/.i/, L.l/.r, dorsal ami ventral lonf,'ilU(linai 

 muscles; A/es, primitive mesoderm cells; ilfP, teloblast; Ncp/i, protonephros; O, moutli ; 

 Of, cesophagus; R, rectum; S, shell; Sclil, hinge; Si\I.li, S.U.v, posterior and anterior 

 adductors; 5/1, apical plate; Wkr, u'kr. pre- and postoral ciliated bands; ic'.(, cilia of 

 apical ])late. 



in a jelly and are either rich in deutoplasm or are en\-elope(l in a nourish- 

 ing albumen. A few molluscs (e.g., raliidiiia iTcipara) are viviparous. 

 A metamorphosis is of wide occurrence, in wliich a vdigcr larva csca])es 

 from the egg (fig. 313); in this can be recognized head, foot, ami mantle, 

 even when one or the other of these is lacking in the atlult. Tlus shows 

 that the fre(iuent absence of mantle, slrell, or head is not a primitive 

 condition, but can only be explained by degeneration. The name 



