* Elasmobranchii have a cartilaginous skeleton, five to seven-gill slits on each side of their head, and a spiracle (a small opening behind each eye). They include sharks, rays, and skates.
* Holocephali also have a cartilaginous skeleton, but they only have one gill slit on each side of their head and no spiracles. They include ratfish and chimaeras.
Here is a table summarizing the key differences between elasmobranchii and holocephali:
| Feature | Elasmobranchii | Holocephali |
|---|---|---|
| Skeleton | Cartilaginous | Cartilaginous |
| Gill slits | Five to seven on each side of head | One on each side of head |
| Spiracles | Present | Absent |
| Examples | Sharks, rays, skates | Ratfish, chimaeras |
Elasmobranchii and Holocephali are both important groups of fish, and they play a vital role in the marine ecosystem.