The Origin of Vertebrates

by Walter Holbrook Gaskell

The skeleton of Petromyzon (Fig. [52]) consists of a cranio-facial skeleton composed of a cartilaginous unsegmented cranium, with the basal trabeculæ and parachordals and a series of branchial and visceral cartilaginous bars forming the so-called branchial basket-work; to these must be added auditory and nasal capsules. In contradistinction to this elaborate cranio-facial skeleton, the spinal vertebral skeleton is represented only by segmentally arranged small pieces of cartilage formed in the connective tissue dissepiments between segmented sheets of body-muscles (myotomes).

Fig. 52.—Skeleton of Petromyzon. (From Parker.)

na., nasal capsule; au., auditory capsule; nc., notochord.

But Petromyzon is derived from Ammocœtes by a remarkable process of transformation, and a most important part of that transformation is the formation of new cartilaginous structures. Thus we see that in Ammocœtes there is no sign of a cartilaginous vertebral column; at transformation the rudimentary vertebræ of Petromyzon are formed. In Ammocœtes the brain-case is a simple fibrous membranous covering; at transformation this becomes cartilaginous. In Ammocœtes there are no cartilaginous structures corresponding to the sub-ocular arches; these are all formed at transformation. It follows, that we can trace back the bony skeleton of the vertebrate head to the skeleton of Ammocœtes, and we may therefore conclude that the primitive cartilaginous skeleton of the vertebrate consisted of the following structures (Fig. [53], B), viz. the branchial bars forming a basket-work, the trabeculæ and parachordals, the auditory and nasal capsules—a clear proof that the cranial skeleton is older than the spinal. Of these structures the branchial bars are the only evidently segmented parts.

Fig. 53.—Comparison of Cartilaginous Skeleton of Limulus and Ammocœtes.

A, Diagram of cartilaginous skeleton of Limulus. Soft cartilage, entapophysial ligaments, deep black; branchial bars simply hatched; hard cartilage, lateral trabeculæ of entosternite, netted; Ph., position of pharynx.

B, Diagram of cartilaginous skeleton of Ammocœtes. Soft cartilage, sub-chordal cartilaginous bands, deep black; branchial basket-work (first formed part), simply hatched; hard cartilage, cranio-facial skeleton, trabeculæ, parachordals and auditory capsules, netted; Inf., position of tube of infundibulum (old œsophagus).

The Soft Cartilage of the Branchial Skeleton of Ammocœtes.