322 MOLLUSCA 



nected in front with the pericardium by a ciliated canal, the nephrostome, 

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

 region of the inner cavity of the inner gill. In this way a connection is 

 established from the pericardium to the exterior, the apparatus being 

 apparently a true nephridium. In primitive forms 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. 322) begins with a short oesophagus, 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 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. 312) are uncommonly wide 

 apart. The two brain (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 supracesophageal 



commissure. The posterior ganglia 

 (united parietal and visceral ganglia) 

 lie near the anus, ventral to the 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. GlochidiumoiAnodonta palpi are also highly sensory, while two 

 (from Balfour) ad, adductor; by, ^ osphradia occur at the basis of 



byssus; s, sense hairs; sn, shell. 



the gills. When eyes occur they are, as 



in the scallops (Pectinida?) , arranged in a row like pearls on the margin 

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

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

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



Veligers (fig. 313) are very common in development. When this stage 

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

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

 adult by a byssus thread, by only a single adductor, and by a hook or tooth on 

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

 themselves by means of the hooks to passing fish, where they produce an ulcer 

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

 the definitive form. After this metamorphosis they fall to the bottom, to live 

 henceforth half buried in the mud. 



