Niddah 43: Ritual Defilement by Sheretz or Shichvat Zeva

The Gemara teaches us about different types of tumah, ritual impurity.  

• the smallest amount of semen causes ritual impurity imparted by shichvat zera 
• coming into contact with a sheretz, a creeping animal, by just the size of a lentil is required for one to become tameh
• Sheretz are small creatures that crawl including rodents, lizards, insects, small creatures that crawl
• The smallest of the eight animals called sheretz must be at least the size of a lentil at birth
• Shichvat zera is "divided in its ritual defilement", where its laws apply only to Jews
• in the laws of sheretz a "land mouse" imparts ritual impurity but a "sea mouse"* does not
• Thus laws of shichvat zera apply to adults
• The laws of shichvat apply to new born creatures and adults

*possibly a snail or a fish that looks like a mouse.