One kind of fish, the hagfish, is considered an invertebrate by scientists. Why is this fish an invertebrate whereas all other fish are vertebrates?
The Myxini are unique among living chordates in that they have a partial cranium (skull), but no vertebrae, and so they are not truly vertebrates. The skeleton is composed of cartilage, and lacks bone.
http://www.seasky.org/deep-sea/atlantic-hagfish.html
As a note on the question, the classification of Hagfish as invertebrate is under dispute, and far from agreed on. I am surprised your instructor moved you into this area of dispute.
4. One kind of fish, the hagfish, is considered an invertebrate by scientists. Why is this fish an invertebrate whereas all other fish are vertebrates?
The hagfish is a unique type of fish that is considered an invertebrate, unlike most other fish which are vertebrates. The distinction between vertebrates and invertebrates lies in the presence or absence of a backbone or vertebral column.
To understand why hagfish are classified as invertebrates, we need to examine their anatomy. While most fish possess a well-developed backbone or vertebral column, the hagfish lacks this structure. It does not have true vertebrae like other fish but rather possesses a primitive notochord, which is a more flexible and less-defined structure.
The notochord is an essential anatomical feature that provides support and rigidity to the body. In vertebrates, it develops into the vertebral column during embryonic development. However, in hagfish, the notochord persists throughout their lives and does not fully develop into a true backbone.
Apart from the absence of a distinct vertebral column, hagfish also exhibit other characteristics typical of invertebrates. They lack jaws and have a simple brain structure. Additionally, they possess a cartilaginous skull, like other fish, but it lacks the well-defined characteristics of a true vertebrate skull.
In conclusion, hagfish are considered invertebrates because they lack a well-developed vertebral column and exhibit other anatomical characteristics more commonly associated with invertebrate animals. This classification is based on their unique anatomy and evolutionary traits.