MOLLUSCA 



229 



the majority in salt water. The organs of respiration arc there- 

 fore gills as a rule, though they have very diverse forms in the 

 different classes; in the terrestrial Mollusca there is a sorl 

 lung. 



There are several sets of special sense organs, notably tenta- 

 cles, which usually serve as organs of touch, olfactory organs, 

 otocysts, and very frequently eyes, which in some genera attain 

 a very high degree of development. Of the internal organs we 

 need merely note the nervous system, which is formed on rather 

 a different type in most cases from that in the Annelida and 

 Arthropoda. Typically there is a pair of cerebral ganglia in 

 the head, or dorsal to the oesophagus ; a second pair, one on 

 each side of the oesophagus, the pleural ganglia; a pair in the 

 foot, the pedal ; and a pair in the region of the viscera, the vis- 

 ceral ganglia. The two ganglia in each pair are connected by 

 nerve fibers called commissures, and each of the three last pairs 

 is connected with the cerebral ganglia by means of nerve fibers 

 called connectives. While reproduction is always sexual, her- 

 maphroditism is very common, and the single germ gland then 

 present is called the ovotestis. The development often takes 

 place through a larva, which at first resembles the trochosphere 

 of the Polychaeta, and is usually provided with a delicate shell. 

 The Mollusca are divided into five classes. 



CLASS I. AMPHINEURA 



The Amphineura (Gr. a\x§i, on both sides, and vevpov, nerve) 

 are marine Mollusca with complete bilateral symmetry and a 

 rather elongated body ; the mouth is anterior, the anus pos- 

 terior. The nervous system has no definite ganglia, and there 

 are few sense organs. The class is divided into two readily dis- 

 tinguishable orders. 



Order 1. Solenogastres 



The Solenogastres (Gr. crooXijv, groove, and yacrrrip, belly) 

 live in che sediment at the bottom of the ocean, generally at 

 considerable depths (Fig. 230). They are more or less cylin- 

 drical, vermiform animals, ordinarily from two to five or six 

 centimeters long, without any shell, but with calcareous spicules 



