PHYLUM MOLLUSCA 245 



ventricle through a common aorta which divides into two branches, 

 one of which supplies the head and foot, and the other carries blood 

 to the visceral hump. The terminal branches of these arteries com- 

 municate with a hemocoele or series of sinuses. Veins carry the 

 blood from the hemocoele to the mantle walls where it is purified 

 and then passed through the pulmonary vein to the single auricle 

 and on into the ventricle. 



Nervous System 



Encircling the esophagus is a ring of nerve tissue which includes 

 three pairs of ganglionic swellings: the cerebral ganglia, situated 

 above the esophagus, supply nerves to the anterior regions of the 

 body; the pleural, pedal, and visceral ganglia lie below the esopha- 

 gus and are connected to the cerebral ganglia by commissures. 

 From them, nerves extend out to the visceral hump and basal parts 

 of the body. The arrangement of ganglia and their connectives is 

 of taxonomic importance. 



Excretory 



The kidney is a yellow gland situated near the heart. Its ureter, 

 a thin-walled tube, parallels the rectum and opens near the anus. 



Reproduction and Life Cycle 



Most fresh-water and terrestrial pulmonate snails, as well as the 

 sea slugs, are hermaphroditic. The majority of the marine shelled 

 gastropods and our fresh-water branchiates, such as Pleurocera, 

 Goniohasis, and Amnicola are unisexual. The reproductive system 

 of a unisexual snail is relatively simple but is exceedingly complex 

 in the hermaphroditic species. 



In bisexual (hermaphroditic) snails cross-fertilization ordinarily 

 occurs. The ova, as well as spermatozoa, are produced by the ovo- 

 testis. Some snails are protogynous, since the ovotestis functions 

 first as an ovary and later as a testis ; others are protandrous, since 

 male gametes are first formed, followed by the production of ova. 

 Spermatozoa pass from the hermaphroditic duct into the sperm 

 duct, and enter the vas deferens which terminates in a muscular 

 penis. By means of the latter organ sperm are transferred into the 

 seminal receptacle of another snail. Ova are passed from the ovo- 

 testis into the hermaphroditic duct, and from there into the oviduct 



