298 



COLLEGE ZOOLOGY 



OJI 



^i:::-//0^' Lifhophaga, ' ''• 

 ■X>\-^v;/^:Jv;^ Rock -bo ring mussel 



^Foot 



Figure 187. Some marine bivalves showing adaptations. Mytilus. the edible mussel, attached 

 by byssal threads to a wooden pier. Mya, the mud clam, a burrowing form with a long siphon. 

 Contrary to a general misconception, the "head" end of a clam is the end opposite the siphons. 

 The walrus is said to feed almost entirely on this clam. Yoldia is capable of leaping through 

 the water; note the united retractible siphons. Lithophaga, the rock-boring mussel, is said to 

 secrete an acid to dissolve the rock into which it burrows. 



type, and no true head. The cephalopods 

 have become free-swimming animals with 

 the foot modified into prehensile tentacles 

 and with the brain and sense organs highly 

 developed. The dominant view of the rela- 

 tion of the mollusks to other phyla is based 

 on the presence of the trochophore lar\'ae 

 among both mollusks and annelids, which 

 indicates that these may have been derived 

 from the same ancestral type. Some of the 

 fossil relatives of Neopilina may well have 

 been the connecting link between the mol- 

 lusks and the segmented annelid worms and 

 arthropods. 



RELATIONS OF THE 

 MOLLUSKS TO MAN 



Most of the mollusks may be considered 

 beneficial to man; some are not. Slugs are 



sometimes injurious in greenhouses and 

 gardens; shipworms {Teredo, Fig. 188) bur- 

 row into the bottom of wooden vessels, 

 wharfs, and piles, weakening and destroying 

 them. Some of the larger octopuses (Fig. 

 186) have the reputation of killing human 

 beings, but are probably not as black as 

 they are painted. Octopuses are very good 

 food; many people prefer them to oysters. 

 They are clean animals; but, because of 

 prejudices, Americans do not use them for 

 food as much as do other peoples, with the 

 exception of one species found along the 

 southern coast of California, which is much 

 sought after for food. 



The value of mollusks as scavengers is 

 little appreciated. The fresh-water clams, 

 for example, are continually ingesting or- 

 ganic particles and thus purifying the water 

 in which they live. Mollusks, however, are 



