THE PHYLUM MOLLUSCA 



251 



washed ashore where they are found and are known as "mermaids' 

 necklaces." 



108. Other Gastropods 



Prosobranchia. Snails are divided into three large orders. Busycon 

 belongs to the order Prosobranchia, in which torsion brings the origi- 

 nally posterior gills, anus, etc., around to the anterior side. Although 

 Busycon has only the left nephridium, gill and auricle, other members 

 of the group, such as the prized abalone of the west coast, have these 

 organs in pairs. It is reasonable to suppose that the abalone represents 

 the primitive condition, and that the loss of organs in such snails as the 

 whelk is an adaptation to the twisted shape of the body. Most of the 

 Prosobranchia are marine, although a number of small forms are found 

 in fresh water. On isolated tropical south Pacific islands some have be- 

 come terrestrial. 



Op/sfhobranc/i/o. In a second large order, torsion as an embryo- 

 logic event is less extreme and may not occur at all. The gills (if present) 

 remain posterior, or at most are moved to the right side, giving the 

 order its name, the Opisthobranchia. Since all members of this group 

 have a single nephridium, gill and auricle, their incomplete torsion is 

 believed to be secondary. These are almost entirely marine, and include 

 some strange forms. 



One group has left the bottom and swims as plankton in the 

 upper water of the open oceans. Each side of the foot is expanded as 

 a muscular flap (Fig. 14.8 A) suggesting wings. Hence their name, the 

 pteropods. Pteropods hang shell down in the water, swimming upward 

 by flapping the "wings," and falling more gently while gathering food 

 from the water. They sometimes form immense swarms and serve as 

 food for whales. 



A second group of opisthobranchs have lost the shell. With it the 

 mantle cavity and original gill have also disappeared, to be replaced by 

 new gills on the back. These are the nudibranchs or sea slugs (Fig. 

 14.8 B). Many of them, particularly those on the west coast, are 

 brightly colored, crawling with great agility over the hydroids and 

 algae upon which they feed. Nudibranchs that feed on hydroids do so 





ABC D 



Figure 14.8. Opisthobranchia: A, a pteropod; B, a nudibranch. Pulmonata: C, a 

 slug; D, a garden snail. {A and C after Parker and Haswell. B and D after Lankester.) 



