36 LAND AND FRESH-WATER SHELLS. 



the mantle-cavity between the pulmonary vein and the heart, and 

 communicates with the pericardium by a ciliated canal, the reno- 

 pericardial canal or nephridial tube^ so that the pericardial cavity 

 is in communication with the exterior through the ureter or duct 

 of the kidney. The ureter runs alongside of the rectum in 

 Helix, and opens near the anus into the mantle-cavity ; in Arion 

 the kidney has " a simple round opening." The kidney is made 

 up of two parts, one thin-walled and sacculated. the other 

 yellowish, and possessing a lamellated structure. The excretion 

 contains ammonium and calcium urate ; and in Hyalina free 

 uric acid and guanin in addition to these. If minutely examined, 

 the orifice of the duct of the kidney in Helix will be seen to be 

 carried as a groove the excretory groove over the anus, ending 

 near the respiratory aperture. 



THE NERVOUS SYSTEM. Lying round the oesophagus is a cord 

 of nervous tissue the circum-cesophageal mass which is divided 

 into that portion which lies above the oesophagus (supra-cesopha- 

 geal ganglia) and that portion which lies below the oesophagus 

 (sub- or infra-oesophageal ganglia). The supra-cesophageal ganglia 

 are two in number, and are united together in the middle line by 

 a band of nerve-fibres (commissure) ; from them nerves pass to 

 nnervate the lips, tentacles, and the buccal mass. The infra- 

 oesophageal ganglia are divided into two portions an anterior 

 portion (pedal ganglia] and a posterior portion (viscero-pleural 

 ganglia) by the passage of the anterior aorta through the centre 

 of its mass. The pedal ganglia give branches to the muscular 

 substance of the foot ; the viscero-pleural ganglia send branches 

 to the body-wall and viscera. A miscroscopical examination of a 

 ganglion shows it to consist of nerve-cells generally unipolar, some- 

 times bipolar and multipolar, placed round a central mass of nerve- 

 fibres derived from them, and arranged in a reticulated manner. 

 To this central mass Leydig has given the name of Punkt-substanz . 



The nerve to the tentacle arises from the supra-cesophagea! 

 ganglion with the labial nerves, and ends at the summit of the 

 tentacle by a knob which consists of nerve-cells and nerve-fibres r 

 surrounded by a muscular sheath derived from the retractor 

 muscle of the tentacle, and which is thought to be olfactory in 

 function, since on excising it on both sides the snail no longer 



