288 PRACTICAL LESSONS IN SCIENCE. 



locomotion, his position while lying in the mud, and the currents 

 of water when the siphons are in action. Water snails and land 

 snails having univalve shells are also interesting to keep for ob- 

 servation, as are also garden slugs or snails that have no shells, 

 or only little scales on the dorsal surface. 



The nervous system of this group consists of three pairs of 

 ganglia, the cervical, pedal and visceral. The sense organs are 

 but slightly developed. Usually the sexes are separate, but 

 hermaphrodite individuals occur. They are generally oviparous, 

 but the fertilized eggs are often retained in the pouches of the 

 parent until the embryo is well advanced. 



The Ostridse, or oysters, belong to this class. They have a fixed 

 left valve; the right valve closes upon the oyster held in position 

 by a strong ligament. They have no siphons, and only a rudi- 

 mentary foot. Oysters are either mainly hermaphrodite indi- 

 viduals or females. The fertilized ova, called spat, after escaping 

 from the organs of the parent, swim freely for several days, then 

 settle on some support as sticks, stones, old shells or other ob- 

 jects, and become fixed. The natural oyster beds of the Old 

 World and many of those of the United States have been ex- 

 hausted, and artificial beds have been made and cultivated and 

 protected to such an extent that oyster culture has become an 

 extensive and profitable industry. 



The Gasteropoda, have a distinct head, often with tentacles 

 and stalked eyes, and simple mantle that secretes a plate-shaped 

 or coiled shell. The muscular foot forms the ventral side of the 

 body, hence the name signifying stomach foot. Eyes, olfactory, 

 tactile and auditory organs are fairly well developed. Sometimes 

 the mantle forms a long siphon. This family includes the uni- 

 valve land snails and the fresh water and salt water univalve 

 snails or mollusks, as well as some that do not secrete a shell. 



The Cephalopoda have a circle of arms, bearing suckers round 

 the mouth, and they have a funnel-shaped perforated foot. The 

 water used for respiration, violently driven from the funnel, tends 

 to force the animal backward. This method of locomotion is 

 common among the animals of this class. The octopus, the 



