358 



THE OSTRACODERMS. 



The prevailing position of the lateral line organs on the neural surface of the 

 head, where they are out of touch with the food or surrounding objects, is only- 

 intelligible on the assumption that they represent the remnants of the gustatory 

 and tactile organs that were located on the neural surface of the head, in their 

 arachnid-like ancestors. See Chapter VII, p. 121. 



The ostracoderms may be divided into the following orders: 



Fig. 235. — Cephalic bucklers of Ostrocoderms and marine arachnids. A, Head of Thyestes; B, Odonto- 

 cephalus; C, Corycephalus ; D, Eukeraspis. Silurian. 



I. ASPIDOCEPHALI. 



Head, thin, broad. Trunk and tail, narrow and small. Exoskeleton form- 

 ing a continuous cephalic buckler; trunk plates separate, segmehtally arranged. 

 Outer surface of dermal armor, smooth, or with low rounded tubercles on poly- 

 gonal areas, that may separate into small, five- or six-sided plates. Marginal and 

 central openings on the dorsal mesocephalic shield, filled with loose polygonal 

 plates belonging to the outer layer only. Olfactory, or hypophyseal opening, not 

 enclosed in the orbital foramen. 



Cephalaspidse. — The head is shield-shaped, rounded or pointed in front, and 

 with thickened margin; posterior margin expanded, cornuate. Oral region is a 

 small membranous area in the center of the thin, convex, haemal wall. The 

 branchiocephalic shield, small, indistinctly segmented; haemal branchial shield 

 absent. Gills not enclosed in an atrial chamber. Large cephalic appendages 



