ANCIENT ANIMALS 



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detailed anatomy not only of the cephalaspid skeleton, but also of the 

 brain and nerves, sense organs, branchial system, and other parts, so that 

 these ancient organisms are actually better known than many living 

 forms. The body was composed of a large "head" and a more slender, 

 scale-armored tail. The "head" was crescentic in outline, convex above 

 and flat beneath, and included what would be the shoulder region in later 

 vertebrates. The upper surface was covered by a solid bony shield, in the 

 center of which, over the brain, were two large eyes, a smaller median 

 pineal eye, and in front of the latter an olfactory pit. Along each margin 

 of the head shield and in the mid-line behind the eyes were specialized 



Fig. 27.4. The ostracoderm Cephalaspis, one of the earliest vertebrates. A, side view. B, 

 dorsal view. C, ventral view. D, head region dissected from below to show the gill chambers. 

 (Redrawn from Romer, by permission The Williams & Wilkins Company.) 



areas which, from their structure, position, and enormous motor nerves, 

 have been identified as electric organs analogous to those of the electric 

 eel. On the flat under side of the head there was a slitlike anterior mouth 

 without jaws, a pulsating scale-covered throat, and along the sides of the 

 throat nine or ten pairs of gill openings. Most of the interior of the "head" 

 was occupied by a series of very large gill chambers. The brain was com- 

 plex, with all the characteristic vertebrate parts; and, as in modern 

 lampreys, the organ of balance had only two semicircular canals, instead 

 of the three of higher vertebrates. 



The jawless mouth and great gill chambers show that the ostracoderms 

 were food strainers, like their even more primitive modern relatives, the 

 lancelets (Fig. B.26) and sea squirts (Fig. 27.5). Essentially they con- 

 sisted of two parts — a pharyngeal gill apparatus for straining food par- 



