The reasons are:
- the blood is very low on nutrition
- the leech is not feeding itself regularly
The leech slows it metabolism a lot, so it can survive on very low nutrition. The blood is very low on nutrition, and it is the leech's food source, so the evolution made it preserve energy.
The leach doesn't have the opportunity to feed itself on regular basis, so it takes maximum advantage of any opportunity and consumes as much blood as possible so that it can survive long term when it will not have food at disposal.
Vascular systems are important to transport materials throughout the organism.
In order to move you need oxygen and water to form the energy to function. The frog is basically drinking water and doesn’t need to move while doing so. You only need to breath to allow your body to move.
Closer to a dog though has some cat qualitys XD<span />
