What is the difference between ray-finned fish and a hagfish?

Ray-finned fish and hagfish are classified in separate classes under the subphylum Vertebrata. Here are their differences:

1. Class: Ray-finned fish belong to the class Actinopterygii, which is the largest class of vertebrates, while hagfish belong to the class Myxini, a small and distinct group of jawless fish.

2. Skeletal Structure: Ray-finned fish have a bony skeleton, with a backbone composed of vertebrae, and their fins are supported by bony rays. In contrast, hagfish have a cartilaginous skeleton made of a soft, flexible material, and they lack paired fins, vertebrae, and jaws.

3. Jaws and Mouth: Ray-finned fish possess jaws and a hinged mouth that allows them to open and close their mouths. Hagfish, on the other hand, are jawless and have a unique "sucking" mouth with rasping teeth that they use to attach themselves to and feed on other animals.

4. Cranium and Brain: Ray-finned fish have a well-developed cranium that encloses the brain, which is divided into distinct regions with specialized functions. Hagfish have a primitive brain and lack a well-organized cranium.

5. Skin: Ray-finned fish have scales that cover their bodies and protect them from damage and infection. Hagfish, conversely, have smooth and scaleless skin that secretes slime as a defense mechanism.

6. Habitat: Ray-finned fish occupy various aquatic habitats, ranging from freshwater rivers and lakes to marine environments, while hagfish are exclusively found in saltwater environments, mainly in deep waters of temperate and cold oceans.

7. Feeding: Ray-finned fish exhibit diverse feeding habits, including carnivory, herbivory, and omnivory, and they use their jaws to capture and ingest food. Hagfish have a unique feeding strategy, acting as scavengers and parasites by attaching themselves to live or dead prey and feeding on their body fluids and tissues.

8. Reproduction: Most ray-finned fish reproduce through external fertilization, where eggs are released into the water and fertilized by sperm. Some species also exhibit internal fertilization. Hagfish reproduce through internal fertilization, with males depositing spermatophores into females, who then store the sperm until the eggs are ready for fertilization.

9. Evolutionary Origin: Ray-finned fish are considered more derived or advanced in evolutionary terms compared to hagfish. They diversified and gave rise to numerous species, including various familiar groups like tuna, salmon, and goldfish. Hagfish, on the other hand, are believed to be more ancient and have retained primitive anatomical features.

In summary, ray-finned fish and hagfish differ significantly in their skeletal structure, feeding mechanisms, mouth and jaw anatomy, habitat preferences, and evolutionary origins, highlighting their distinct evolutionary histories and adaptations to different ecological niches.