For example, we find that the Gemara contrasts between the ritual impurity imparted by shikhvat zera – semen – and that of a sheretz – a creeping animal. While secreting even a tiny amount of semen would render a person impure, when coming into contact with a sheretz, a minimum size of a lentil is necessary for the person to become tameh.
Generally, any one of rodents, lizards, insects or other small creatures that crawl would be considered a sheretz. Ritual impurity is imparted by the carcasses of eight creeping animals (Vayikra 11:29-37). The Sages stated that the smallest of these eight animals was at least the size of a lentil’s bulk at birth. Therefore, one only contracts ritual impurity if he comes into contact with a piece of the carcass of a creeping animal no smaller than that size.
The Gemara quotes a baraita that teaches that another difference between shikhvat zera and sheretz is that shikhvat zera is “divided in its ritual defilement” while sheretz is not. In explaining this, the Gemara says that it cannot mean that the laws of ritual defilement of shikhvat zera apply only to Jews and not to non-Jews, since we find a similar division in the laws of sheretz; a “land-mouse” imparts ritual impurity, while a “sea-mouse” does not. The Gemara concludes that the ritual purity laws of shikhvat zera apply only to adults, while the laws of sheretz are applicable even to new-born creatures.
We know that a “land mouse” is mus musculus – an ordinary house mouse. There is some discussion about how to identify a “sea mouse.” Some suggest that it is a type of snail, while Rashi argues that it is a fish that is similar in appearance to a mouse. According to the Gemara in Massekhet Hullin (daf 126b ) the Torah limits the laws of sheretz to animal that live on the land (see Vayikra 11:29), excluding water-based animals.