The Vertebrata are defined as animals with a backbone, but this is not exactly correct. For in the “round-mouthed” eels (Cyclostomes) including the lamprey and hagfish the axial skeleton is a cylindrical rod or cord (the notochord) consisting of somewhat gelatinous internal substance enclosed by a sheath of tough fibrous tissue.
The notochord itself is quite unsegmented. Its fibrous sheath extends upwards on either side to enclose the dorsal nerve cord (spinal cord). In the neural arch of fibrous tissue formed in this manner minute rods or plates of cartilage are embedded. The little cartilages have been interpreted as vertebrae, but they are merely rudiments of them.
Thus, here are animals without a “column”, i.e., a longitudinal series of definitely formed vertebrae articulated to one another, and yet they are called vertebrates.
The fact is that the vertebral column is not the most important feature of the animals called vertebrates, it is, however, one of the characteristics of the most animals of the group. There arc six fundamental characteristics of the vertebrate types and there are a few others which are also fundamental but perhaps less distinctive. This enumeration is only arbitrary, and can serve only to facilitate the work of a student. As his knowledge deepens, he will be able to free himself from the limitations which such a numerical classification may have put on his understanding.
The six most important characteristics are:
1. The pharynx and pharyngeal structures (gill-clefts, etc.).
2. The notochord or structural axis.
3. The tubular central nervous system.
4. The ventral heart.
5. Position of mouth.
6. Division of the coelom into:
(a) dorsal segmented part comprising cavities of the somites.
(b) ventral unsegmented part (splanchnocoel), which is subdivided by the transverse septum into a thoracic and an abdominal portion.
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