220 



ECTOPROCTA THROUGH ECHIUROIDEA: 



some feed upon dead plants and animals or upon liv- 

 ing animals. Land gastropods include two groups, 

 the snails and the slugs. Most snails inhabit forests 

 where they are sheltered under fallen trees, decaying 

 leaves, rocks, or the soil. The natural food of these 

 animals is vegetation, but some prey upon earth- 

 worms, their own eggs, and each other. Among those 

 that are herbivorous, certain snail species cause 

 considerable damage to crops and garden plants. 



Slugs are more active at night and tend to be more 

 closely associated with man's homes than are snails; 

 hence, slugs probably do more garden damage. 

 Slugs, like snails, prefer plants as food, but also act 

 as scavengers and predators. Few people realize that 

 slugs can hang by a thread made from dried mucus. 

 This reminds one of spiders and their silk, but un- 

 like spiders, slugs cannot travel up their thread. 



Marine gastropods are of many kinds, but most re- 

 semble snails or slugs. Ocean univalves feed on 

 many difiTerent things. The scum of microscopic 

 algae is used by many rock dwellers, especially the 

 limpets. Abalone and sea hares feed upon larger sea- 

 weeds, as do a very few snails and limpets. Sur- 

 prisingly few marine gastropods feed upon detritis 

 which is rich in decaying and living organisms. The 

 tube snail, Aletes, secretes a triangular sheet of 

 mucus with which it nets minute life and decaying 

 material. The moon snails and murexes are two 

 groups of carnivorous snails that use the radula to 

 bore through the shells of other mollusks. The sea 

 slugs have specialized meat diets, some species 

 showing preference for sponges, hydroids, sea pan- 

 sies, bryozoans, ascidians, and other animals. 



Most gastropods are hermaphroditic, and in ma- 

 rine species a free-swimming larval stage occurs 

 between egg and adult form. The slipper-shells, 

 Crepidula, cup-and-saucer shells, Cruabulum: and 

 certain other taxa have a strange life history in which 

 adult life is started as a male, the male later changes 

 to a sexless individual, and the latter in turn trans- 

 forms to a female. In most limpets, abalones, and 

 many snails the sexes are separate. In land and 

 fresh-water species adult development generally is 

 direct from an egg. 



Gastropods are among the more economically im- 

 portant animals. Abalones are a most important 

 source of gastropod food for humans in the United 

 States, and other univalves are consumed elsewhere 

 in the world. Univalve shells have been and are the 

 counterpart of money in many parts of the world. 



For example, olive shells, Olwella, were exchanged 

 as wampum by Pacific Coast Indians. These shells 

 still are very common in many places, so common 

 that anyone familiar with their numbers might 

 wonder what kept all Pacific Coast Indians from 

 being millionaires. The answer is that the economic 

 principles of supply and demand was in efTect. Al- 

 though-a few strings of olive shells might buy a wife 

 inland it probably cost a sack full for a like purchase 

 along the Coast. Hence, units of value varied 

 geographically, but two common measurements of 

 wampum strings were thumb tip to elbow and thumb 

 tip to the base of hand. Indian "businessmen" had a 

 tattoo mark on the thumb side at the base of the hand 

 to indicate the latter-mentioned measurement. Aba- 

 lone also were items of wampum. In addition, it was 

 not uncommon for Pacific Coast Indians to place 

 small abalone shells in the eye-sockets of their dead. 



Gastropod shells were used for other purposes in 

 former times. It is hard to imagine that knowledge of 

 gastropods could make a people a world power, but 

 such was the case with the Phoenecians. The cities 

 of Tyre and Sidon became banking centers and 

 crossroads of the ancient world as a result of Tyrian 

 purple dye extracted from murex snails. This dye 

 was used for the clothing of royalty, idols, Roman 

 senators, and others of high position from the 

 fifteenth century B.C. to the seventh century a.d. 

 Many of our snails possess the same or a similar pur- 

 ple dye, especially certain of our marine snails and 

 sea hares. 



CLASS PELECYPODA (Bivalves) 



Diagnosis: bilaterally symmetrical and unseg- 

 mented; body without a distinct head but with a 

 mouth bounded by flaps (labial palps); radula ab- 

 sent; foot often large, usually wedge-shaped; shell 

 divided into two valves (Figure 13.8). 



The bivalves are clam-like individuals and include 

 such organisms as clams, mussels, scallops, and 

 oysters. Most of them are marine, but there are 

 fresh-water forms. Most pelecypods feed by filtering 

 organisms from the water. They secrete a sticky 

 mucus over the gills and minute organisms stick to 

 this mucus. The water current necessary to bring 

 food to the gills is created by the beating of cilia. 

 The cilia also serve to direct the mucous-trapped 

 food to the mouth. The most surprising exception to 

 the general feeding habit is found in the wood-eating 



