SUBDIVISIONS OF THE BODY. 349 



The ostracoderms probably rose from the merostomes during the Ordovician, 

 and reached their highest development in the upper Silurian, after which they 

 rapidly declined, disappearing at the close of the Devonian. 



The first recognizable ostracoderm to appear in America is Palaeaspis, from 

 the lower Silurian of Perry Co., Penn. (Fig. 244, B and C.) 



Walcott has described fragments of bony plates from the lower Trenton 

 horizon of the Ordovician, Colorado, the primitive character of which is shown by 

 the pronounced lamination of the outer derrtinal layers. 1 But evidence based on 

 such fragments, however well preserved, is inconclusive since, as we have seen in 

 Chapter XVI, there is no way to distinguish fragments of the exoskeleton of a 

 primitive ostracoderm from those of the higher marine arachnids. 



The ostracoderms, like their arachnid ancestors, are small, usually a few 

 inches long. Two isolated species only attain a length of one and a half or 

 possibly two feet. They inhabited shallow waters, and crawled clumsily, oral 

 side down, over or through soft muddy bottoms, or swam heavily, oral side 

 up, with spasmodic strokes of their oar-like cephalic appendages, aided by 

 the more flexible posterior portions of the slender trunk and tail. They probably 

 fed on minute organisms sifted out of the mud or water, or on the soft parts of 

 plants, or on decomposing organic matter. 



Most ostracoderms have large, rounded, or pointed heads, a small trunk, and 

 a tail consisting of a narrow terminal ribbon, or filament, with a ventral lobe some 

 distance in front of the end. 



Subdivisions of the Body. The body may be divided into a procephalon, 

 mesocephalon, branchiocephalon, trunk and tail. 



The procephalon, which may form a narrow projecting rostrum, contains on 

 its neural surface the median and lateral eyes and the olfactory organs. It 

 is intimately united with the mesocephalon, to which belong the oral arches, or 

 jaws, and the oar-like cephalic appendages. 



The branchiocephalon may be separated from the mesocephalon by a distinct 

 hinge joint (antiarcha), or by a transverse furrow or scar (cephalaspidae). It 

 contains six to eight pairs of gills, usually enclosed in a large peribranchial, or 

 atrial chamber, that is covered on all sides, except the posterior, by large 

 dermal plates. The right and left sides of the chamber are continuous on the 

 ventral side, but are separated along the mid dorsal line by the tough tissues that 

 suspend the branchial portion of the head to the inner surface of the branchial 

 shield. The cloaca may open into the posterior part of the peribranchial chamber, 

 its materials being discharged with the water of respiration from the posterior 

 opening. (BothriMepis.) The viscera, stomach, intestines and reproductive 

 organs lie anterior to the cloacal opening, in that part of the head and trunk en- 

 closed within the atrial chamber. (Bothriolepis). In the cephalaspidae there is 



1 Walcott, Bull. Geol. Soc., Vol. III. 



