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). [Illustration: FIG. 52.--SKELETON OF PETROMYZON. (From PARKER.) _na._, nasal capsule; _au._, auditory capsule; _nc._, notochord.] But Petromyzon is derived from Ammocoetes 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 Ammocoetes there is no sign of a cartilaginous vertebral column; at transformation the rudimentary vertebræ of Petromyzon are formed. In Ammocoetes the brain-case is a simple fibrous membranous covering; at transformation this becomes cartilaginous. In Ammocoetes 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 Ammocoetes, and we may therefore conclude {126}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. [Illustration: FIG. 53.--COMPARISON OF CARTILAGINOUS SKELETON OF LIMULUS AND AMMOCOETES. 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 Ammocoetes. _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 oesophagus).] THE SOFT CARTILAGE OF THE BRANCHIAL SKELETON OF AMMOCOETES.