Question

why we discet vertebrate ventrally and invertebrate dorsally

Answer 1

This, along with the ribs attached to the spinal column, makes it difficult to cut into the animals from the dorsal side, if organs is what you're going for. So usually, when students are asked to dissect a vertebrate, it will be cut open from the ventral side.

Answer 2

Answer:

Vertebrates include amphibians, reptiles, mammals, and birds, as well as the jawless fishes, bony fishes, sharks, and rays. More than 64,000 species of vertebrates have been described, but the extant vertebrate species represent only a small portion of all the vertebrates that have existed.