314 



MOLLUSCA 



Even more room in the visceral sac is demanded by the digestive tract in 

 which oesophagus, stomach, a coiled intestine, a voluminous liver, and 

 usually salivary glands may be recognized. The liver is usually a paired 

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

 stores up glycogen, but forms an enzyme (cytase) which converts cellulose 

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

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

 It is a plate or band armed with teeth which lies on the floor of the pharynx- 

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



(figs- 334, 335)- 



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

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



FIG. 313. Veliger larva (trochophore) of Teredo navalh (from Hatschek). A, 

 anus; /, stomach; ./,, intestine; I,, liver; LM.d, LAI.v, dorsal and ventral longitudinal 

 muscles; Mes, primitive mesoderm cells; MP, teloblast; NepJi, protonephros ; O, mouth; 

 Oe, oesophagus; R, rectum; 5, shell; Schl, hinge; SM.h, SM .v, posterior and anterior 

 adductors; Sp, apical plate; Wkr, wkr, pre- and postoral ciliated bands; ws, cilia of 

 apical plate. 



in a jelly and are either rich in deutopksm or are enveloped in a nourish- 

 ing albumen. A few molluscs (e.g., Paludina vivipara) are viviparous. 

 A metamorphosis is of wide occurrence, in which a veliger larva escapes 

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

 even when one or the other of these is lacking in the adult. This shows 

 that the frequent absence of mantle, shell, or head is not a primitive 

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



