Japan during the Yayoi period engaged in intensive rice agriculture in paddy fields introduced from southern China via the Ryukyu Islands which developed a manorial feudal economy similar to that of medieval Europe. As the Yayoi economy used no form of currency, bartering was used to trade goods and services, predominantly farm implements. Yayoi farmers fished, hunted, gathered and farmed. The introduction of a highly advanced form of rice cultivation using irrigation propelled the Yayoi economy. Terraced paddy fields made the Yayoi more successful in the growing of rice that surpluses were often produced. The agricultural surpluses produced by the manorial Yayoi economy stimulated Japan's early handicraft industries and the establishment of urban villages and permanent settlements started to appear in the Yayoi agricultural community as cities didn't exist at that time. As the Yayoi economy became more sophisticated, Japanese craftsmen began to venture in metallurgy and began to develop their own tools such as swords, arrowheads, axes, chisels, knives, sickles, and fishhooks. Decorative items such as ceremonial bells and mirrors were used as religious rituals and status symbols. As the Yayoi population increased, the society became more stratified and complex. They wove textiles, lived in permanent farming villages, and constructed buildings with wood and stone. Yayoi merchants and farmers also accumulated wealth through land ownership and the storage of grain. Such factors promoted the development of distinct social classes. Contemporary Chinese sources described the people as having tattoos and other bodily markings which indicated differences in socioeconomic status. In ancient Japan, a person's wealth was described in "kokus" and measured in bales of rice. One koku was 47 gallons (180 liters) of rice. As rice signified money, large quantities of rice had to be stored and distributed nationally. Moreover, the imperial Japanese tax system was based on rice and it taxed peasants with rice and paid the salaries of high ranking government workers with it. Additional trade goods made from rice such as sake, rice wine, and vinegar were used as commodity money. Soon, rice played an important role in Japan's economy and was used as currency for more than a millennium. Small amounts of trade Japan and China began around 2000 BC when Chinese merchants sailed across the East China Sea to Japan in wooden canoes. The two nations increased their economic relations greatly by the 1st century AD when Japanese envoys were sent to Korea and China. Japanese silk was in high demand by the ancient Koreans and Chinese as it was used produce clothing.