THE EVIDENCE OF THE SKELETON 



125 



structures in the fossils of the Silurian age either among the verte- 

 brate or invertebrate remains. Fortunately for this investigation 

 there are still living on the earth two representatives of that age ; on 

 the invertebrate side Limulus, and on the vertebrate side Ammoccetes. 



The Elasmobranchs represent the most primitive of the gnatho- 

 stomatous vertebrates. Below them come the Agnatha, known as the 

 cyclostomatous fishes or Marsipobranchii, the lampreys (Petromyzon) 

 and the hag-fishes (Myxine). 



The skeleton of Petromyzon (Fig. 52) consists of a cranio-facial 

 skeleton composed of a cartilaginous unsegmented cranium, with the 

 basal trabecule 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 contradis- 

 tinction to this elaborate cranio-facial skeleton, the spinal vertebral 



na 



an 



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

 na., nasal capsule; an., auditory capsule; nc, uotochord. 



skeleton is represented only by segmen tally arranged small pieces of 

 cartilage formed in the connective tissue dissepiments between 

 segmented sheets of body-muscles (myotomes). 



But Petromyzon is derived from Ammoccetes by a remarkable 

 process of transformation, and a most important part of that trans- 

 formation is the formation of new cartilaginous structures. Thus we 

 see that in Ammoccetes there is no sign of a cartilaginous vertebral 

 column ; at transformation the rudimentary vertebras of Petromyzon 

 are formed. In Ammoccetes the brain-case is a simple fibrous mem- 

 branous covering ; at transformation this becomes cartilaginous. In 

 Ammoccetes 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 Ammoccetes, and we may therefore conclude 



