Blastocyst resembles a large red tortoise, with two cannons sprouting form it's yellowish shell. Autopsies show that its shell is in fact hollowed out completely containing only a liquid-like fluid which it fires from its cannons as an attack. It stands on two feet and only gets down on all-fours to fire its cannons.
Contrary to popular belief, Blastocyst is not a water type Pokémon. It is in fact, a recently discovered, "cell type Pokémon." Attempts to wean it (and it's predecessors) to swim have been disastrous, usually resulting in the death of the Pokémon and it's trainer.
It's primary attacks focus on the launching of the fluid which it keeps in it's shell, the most common attack known by all of it's kind, and usually learned its Blastula phase, is called "Fluid Gun." The Pokémon has a number of other attacks which alter the offspring of the opposing Pokémon, which, honestly, mixes things up quite a bit.