364 MOLLUSCA. 



middle line. Each consists of a dorsal smooth-walled chamber 

 and a lower portion traversed by threads, both connected behind 

 but separated elsewhere by a thin partition. The lower chamber 

 is connected in front with the pericardium by a ciliated canal, the 

 nephrostome, while the upper opens to the outside by a short 

 canal, the ureter, the external opening being in the region of the 

 inner cavity of the inner gill. In this way a connexion is estab- 

 lished from the pericardium to the exterior, the apparatus being 

 apparently a true nephridium. In many it serves also as genital 

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

 The animals are usually dioecious, the gonads being acinose 

 glands. 



The digestive tract (fig. 351) begins with a short oesophagus, 

 widens out to a large stomach from which a slender intestine leads, 

 with many convolutions, to the anus. In the majority of Acephals 

 the terminal portion enters the pericardium in front and below, 

 passes through the ventricle and out through the upper posterior 

 wall of the pericardium. In its course the alimentary tract i& 

 enveloped by the gonads and the 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 'crystalline style/ a 

 rod-like structure of uncertain significance. 



The three typical molluscan ganglia (p. 353) are uncommonly 

 wide apart. The two brain ganglia (cerebropleural ganglia) lie 

 either side of the mouth at tho base of the labial palpi and central 

 to the anterior adductor. They are very small, since cephalic 

 sense organs are lacking, and are united by a transverse supra- 

 oesophageal commissure. The posterior ganglia, composed of the 

 united parietal and pedal ganglia, lie near the anus ventral to the 

 posterior adductor. The pedal ganglia, rather far forward in 

 the muscles of the foot, are closely approximate. Of the higher 

 sense organs only the otocysts near the foot are constant. The 

 labial palpi are also highly sensory, while two small osphradia occur 

 at the basis of the gills. When eyes occur they are, as in the scal- 

 lops (Pectinidae), arranged in a row like pearls on the margin of 

 the mantle. Small tentacles with sensory powers may occur both 

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



Veligers (fig. 342) are very common in development. When this stage 

 is lacking the history may contain a metamorphosis as in the fresh-water 

 Anodonta. The young which grow in the maternal gills are known as 

 Glochidia, which are distinguished from the adult by a byssus thread, 

 by only a single adductor, and by a hook or tooth on the free margin of 



