CHAPTER XVI. 

 THE DERMAL SKELETON. 



We recognize four distinct structures in the skeleton of primitive vertebrates: 



1. The dermal skeleton, consisting of bony plates more or less intimately united 

 to form a continuous external armor for the head and trunk; 2. the primordial 

 endocranium, consisting of a broad unsegmented plate of fibrocartilage lying on 

 the hsemal side of the brain; 3. the gill-bars, segmentally arranged cartilage bars 

 lying in the visceral arches; 4. the notochord; 5. neural arches, segmentally ar- 

 ranged blocks, or half-rings of cartilage distributed along the sides of the nerve 

 cord and notochord. These structures differ from one another in their chemical 

 and histological composition, and in their origin. The general trend in the evolu- 

 tion of the vertebrate skeleton is toward the elimination of the two oldest constitu- 

 ents, the dermal skeleton and the notochord, and the union of the remaining 

 elements, without distinction of origin, structure, or previous function, into a 

 common axial skeleton. 



These five sets of skeletal structures are already established in the arachnids, 

 viz: i. The external chitenous armor with its primordial canaliculi and lacunae. 



2. the fibrocartilaginous endocranium or plastron; 3. the cartilaginous bars 

 supporting the branchial appendages; 4. the middle cord, or lemmatochord, 

 and 5. the segmentally arranged cartilages over the spinal cord. Nothing re- 

 sembling this assemblage of skeletogenous tissues is found in any other animals 

 outside the vertebrates and arachnids. 



I. THE DERMAL SKELETON OF VERTEBRATES. 



The dermal skeleton of primitive vertebrates consists of a series of bony 

 plates, not preformed in cartilage, arising in or beneath the epidermis. It has 

 been generally assumed that the most primitive dermal skeleton is one consisting 

 of small isolated placoid scales, similar to those in the elasmobranchs; and that 

 the larger plates seen in the ganoids, dipnoi, and ostracoderms were formed by 

 the secondary union of such scales. This assumption is based on the prevalent 

 belief that the elasmobranchs and cyclostomes are the most primitive vertebrates, 

 and that the above mentioned heavily armored forms were derived from them. 

 This view is untenable, since it takes no account of the fact that the ostracoderms, 

 which are the oldest known, and the most primitive fish-like vertebrates, were pre- 

 eminently heavily armored forms. 



We shall reverse the usual way of approaching this question and start with the 

 assumption that the most primitive vertebrate dermal armor is like that of the 



19 289 



