PHYLUM MOLLUSC A 189 



occupy most of the visceral mass surrounding the ahmentary 

 canal. A gelatinous rod, called the crystalline style, frequently 

 occurs in the stomach. Recent investigations seem to indicate 

 that this organ aids in separating food from foreign particles and 

 probably contains a store of enzymes for use in the digestive 

 processes. 



Circulatory System. — The heart, which lies in the dorsal part 

 of the visceral mass, is surrounded by a pericardium and consists of 

 a single ventricle and two auricles. A system of arteries carries 

 the blood from the heart to the tissues, where they frequently 

 terminate in sinuses. The veins which return the blood to the 

 heart pass first through the excretory organ and then through the 

 gills. Separate vessels supply the mantle with blood and return 

 the blood directly to the heart without passing through the 

 excretory organ or gills. 



Excretory Organs. — The pair of nephridia characteristic of 

 Acephala are frequently called the organs of Bo j anus after their 

 discoverer. They lie immediately below the pericardium. Each 

 nephridium is a wide tube, bent upon itself, one end of which 

 opens into the pericardial cavity and the other, non-glandular in 

 structure, serves as a urinary bladder with its opening usually on 

 a minute papilla into the inner cavity of the gill chamber. 

 Another gland of excretory function, called Keber's organ, lies 

 anterior to the pericardium into which it discharges. 



Nervous System. — A pair of cerebropleural ganglia is located 

 one on each side of the mouth near the base of the labial palpus. 

 A transverse supraesophageal commissure connects this pair of 

 ganglia. Two nerve cords originate in each cerebropleural 

 ganglion and connect this ganglion with the two other nerve 

 centers on each side of the body. One of these passes ventrally 

 to communicate with the pedal ganglion, embedded in the muscles 

 of the foot, and the other passes posteriorly to a region ventral 

 to the posterior adductor where the parietal and viceral gangha 

 of the more primitive molluscs have become fused to form a 

 single posterior ganglion. 



Sense Organs. — There are but few highly specialized sensory 

 organs. Statocysts are frequently found near the pedal ganglia, 

 though they seem to have nerve connections with the cerebro- 

 pleural ganglia. Patches of sensory epithelium called osphradia, 

 and thought to have olfactory functions, are located near the 

 base of the gills. The labial palpi and the edges of the mantle 



