248 



THE ANIMAL KINGDOM 



Manllei 



Shell 



Anus 



rCiliated 

 crown 



Mouth. 



-Anu$ 



Foot Atlu.s-' ^Mouth Mouth-" 



A B C 



Figure 14.4. Torsion in the gastropod Acmaea (a limpet). A, Young larva, showing 

 beginning of shell and foot. B, Just before torsion, with a U-shaped digestive tract. C, 

 Just after torsion (arrow indicates movement that has occurred). All of these stages swim 

 with the ciliated crown uppermost in the water. They are shown here in positions com- 

 parable with that of the adult snail. (After Boutan, 1899.) 



manent. Following this, development is asymmetrical, with the struc- 

 tures of one side often suppressed. The body elongates dorsally, grow- 

 ing up in a spiral pattern. The enclosing spiral shell forms a structure 

 characteristic for the class. 



107. Bus/con 



The familiar large whelks of the eastern seaboard belong to the 

 genus, Busycon, of which B. canaliculatum (Fig. 14.5), about eight inches 

 long, is the most common. Busycon lives on sand and mud, where it can 

 plow about with its large powerful foot searching for small clams and 

 other prey. The mouth is borne on a long, retractile proboscis which is 

 usually withdrawn into the head, but may be shot out quickly to capture 

 food. The teeth of the radula are long, sharp and recurved so that 

 Busycon can not only pierce the flesh of its prey, but draw it into the 

 mouth. 



Food is swallowed through a long esophagus (Fig. 14.5) to a curved 

 stomach lying in the lower whorl of the body. From the stomach an 

 intestine bends dorsally and down the anterior surface of the whorl to a 

 short wide rectum that opens at an anus in the mantle cavity over the 

 head. A pair of salivary glands beside the esophagus secrete juices 

 (probably containing enzymes for digesting carbohydrates) into the an- 

 terior end of the esophagus. The stomach lies between a pair of large 

 digestive glands that occupy most of the space in the upper body 

 whorls. Ducts from these glands open into the stomach. They are not 

 known to secrete digestive juices, but do take up food particles from 

 the fluid that flows up the ducts into the glands, and digest them in food 

 vacuoles. 



The mantle cavity formed by the fleshy mantle that lines the inner 

 surface of the shell surrounds the anterior part of the body. On the 

 left side both shell and mantle are drawn out into a long siphon, a 

 tubular fold through which water is drawn into the mantle cavity. A 

 large chemoreceptor at the base of the siphon samples the incoming 

 water before it passes over the single gill, a flat, oblong, feathery struc- 



