MOLLUSC A. 355 



Nephridia and sexual organs are primitively paired, but fre- 

 quently are single by the degeneration of the structures of one side. 

 The animals are either hermaphroditic or dioecious, but the gonads 

 are always very large. Even more room in the visceral sac is 

 demanded by the digestive tract in which oesophagus, stomach, a 

 coiled intestine, a voluminous liver, and frequently salivary glands 

 may be recognized. The radula or lingual ribbon is also a char- 

 acteristic organ, and its absence from the Acephala is probably to- 

 be explained by 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 communication of food (figs. 366,. 

 367). 



Keproduction is exclusively sexual; budding, fission, or parthen- 

 ogenesis have not yet been observed. The eggs, united in large 

 numbers, are usually enveloped in jelly and are either rich in 

 deutoplasm or are enveloped in a nourishing albumen. A few~ 

 molluscs (e.g., Paludina vivipara) are viviparous. A metamor- 



ties 



FIG. 342. Veliger larva (trochophore) of Teredo navalte. (From Hatschek.) A, anus: 

 J, stomach ; J^ intestine; , liver; LM.d, LM.v, dorsal and ventral longitudinal 

 muscles ; Meg, primitive mesoderm cells ; JfP, teloblast ; Neph, protonephros ; O, 

 mouth ; Oe, oesophagus ; R, rectum ; S, shell ; ScM, hinge ; SM.h, SM.v, posterior 

 and anterior adductors; S, apical plate; Wkr, wkr. pre- and postoral ciliated 

 bands ; uvs, cilia of apical plate. 



phosis is of wide occurrence. In such cases a 'veliger' larva 

 escapes from the egg (fig. 342) ; in this can be recognized head, 

 foot, and mantle, even in those cases where one or the other of 

 these is lacking in the adult. This shows that the absence of 



