154 



MOLLUSCA 



spiral is a left-handed one), which attaches its egg capsules to sticks 

 and leaves in the water. Either form is excellent for the study of 

 embryonic development, since the eggs are deposited in laboratory 



aquaria and develop rapidly. (Figure 70, A 

 and 5.) 



The land slugs {Limacidae) have a vestigial 

 shell. A species found in California reaches the 

 length of twelve inches. The giant slug {Arioli- 

 ?nax sp.) is used by the South American Indians 

 in the manufacture of " bird lime " to capture 

 Fig. 70. Left, hummingbirds. 



Lymnaea with dex- om c ^ '^ j J*1*~ 



^ , , ,, „. , ^ Snails are of great sanitary and medical sig- 



tral shell. Right, , r . 1 j •.• 



Physa with sinistral nincance as hosts ot larval trematodes, parasitic 



shell. in vertebrates, including man. (See p. 77.) 



Class 4. Scaphopoda. — The tooth shells or 



tusk shells have a tubular calcareous shell open at both ends. The 



foot is at the larger end. A lingual ribbon (radula) is present, and 



the animal has a rudimentary head. Tentacles, eyes, a heart and 



gills are absent. The captacula are ciliated contractile filaments 



perhaps for breathing and securing food. The animal has the 



univalve shell and radula of the Gastropoda and the symmetry 



and pointed foot, without tentacles or head, that characterize the 



Lamellibranchiata. 



The Dentalium or iusk shell was used in California among the 

 Indians and the early whites as currency. The shells used were 

 valued at ^5.00 each if about two and one-half inches long; smaller 

 ones were about one inch long and were worth about ^oi. An eleven- 

 shell string was worth about I50.00. Dentaliums were traded for 

 wives, clothing, furs, and woodpecker scalps, whose red topknots 

 were of considerable value also. 



Class 5. Cephalopoda. — The Cephalopoda, which include the 

 squids, sepias and octopuses, are highly developed marine Mollusca. 

 They have a true head, with well-developed eyes and olfactory 

 organs, and the anterior portion of the foot is modified into tentacles 

 or arms. The body is bilaterally symmetrical. Locomotion is 

 accomplished by movements of the tentacles and by expulsion of 

 water from a funnel or siphon leading out from the mantle cavity. 

 The shell is usually internal, that of the cuttlefish being sold as 

 " cuttle-bone," but in the Nautilus, the shell is highly developed as 

 a chambered external coiled structure. 



