MOLLUSCA 



nccled in front \vith llie pericardium by a ciliated canal, the nephrostonie, 

 while the upper opens to the outside by a short canal, ihe ureter, in the 

 region of the inner cavitv of the inner gill. In tliis wav a connection is 

 established from the pericardium to the exterior, the apparatus being 

 apparently a true nephridium. Iir primitive forms it ser\es also as 

 genital duct, but usually the genital and reproduclixe ducts are separate. 

 The animals are usually dio?cious, the gonads being acinose glands. 



The digestive tract (lig. 322) begins with a short crsophagus, widens 

 out to a large stomach from which a slender intestine leads, with many 

 convolutions, to the anus. In most Acephals the intestine passes through 

 the pericardium and ventricle. The alimentary tract is en\clopod by the 

 gonads and tlie voluminous liver, the secretion of the latter emptying 

 by two ducts into the stomach. Usually the stomach has a blind sac, 

 in which lies the cryslaUiiic style, a rod-like structure of uncertain 

 signilicance. 



The three typical moUuscan gangUa (p. 312) are uncommonly wide 

 apart. The two br;un (cerebropleural) ganglia lie either side of the mouth 

 at the base of the labial palpi. They are very small, since cephalic sense 

 organs are lacking, and are united by a transverse suprao-sophageal 



comniissure. The posterior ganglia 

 (united parietal and visceral ganglia) 

 lie near the anus, ventral to tlie poste- 

 rior adductor. The pedal ganglia, 

 rather far forward in the muscles of the 

 foot, are closely approximate. Of the 

 higher sense organs only the statocysts 

 near the foot are constant. The labial 

 Fig. 325. — C.lochidiuinof .1h(ji/o;//'i7 palpi are also highly sensory, while two 



(from Balfour), ail. adiluclor; /)v, 

 byssus; .■;, sense hairs; s//, shell. 



mall osphradia occur at the basis of 



the gills. When eyes occur they are, as 



in the scallops (Pcclinida-), arranged in a row like pearls on the margin 



of the mantle. Their structure is different from that of the cephalic eves 



of other molluscs. .Small tentacles with sensory powers may occur on 



the margin of the mantle and the tip of the siphon. 



Vcligers (fig. 313) arc very common in development. When this stage 

 is lacking the history may conlain a metamorphosis as in Ihe fresh-water mussels. 

 The young, known as Glochidia, live in the maternal gills and differ from the 

 adult by a by.ssus thread, by only a single adductor, and bv a hook or tooth on 

 the free margin of the shell (tig, 325). After escape from the gills they attach 

 them.selves by means of the hooks to ])assing fish, where thev produce an ulcer 

 in the skin in which they grow, and by deveiojjing the adductor muscles attain 

 the definitive form, .'\ftcr this metamorphosis they fall to the bottom, to live 

 henceforth half buried in the mud. 



