424 THE CHANGING GENERATIONS 



tides from the stream of water entering the mouth, and for respiration; 

 and a locomotor tail equipped with muscles, nervous system, and sense 

 organs, to transport the gill system from place to place. Most ostracoderms 

 were sluggish bottom dwellers, as shown by their flattened form. They 



Fig. 27.5. Model of the tunicate or sea squirt Cynthia, a primitive marine chordate distantly 

 related to the acorn worms, Amphioxus, and Cephalaspis. A stream of water is pulled in 

 through the mouth (above) by the beating of cilia that line the walls of the enormousb 

 developed pharynx. The perforated walls of the pharynx strain plankton from the water 

 and carry it to the funnel-like esophagus. The stomach (not here visible) lies beneath, with 

 the tubular intestine looping forward and rising toward the lateral excurrent opening. The 

 gonads can be seen draped over the intestine, their slender duct rising alongside of the 

 latter. The circulatory system is unique; the ventral heart first pumps blood into the 

 pharyngeal walls for aeration, then reverses its action and pumps the blood into the remain- 

 ing viscera. The wrinkled brown covering or "tunic" that encloses the body is responsible 

 for the name tunicate. {Courtesy American Museum of Natural History.) 



served as the chief food organisms of the larger and more powerful 

 eurypterids, and their bony armor and electric organs were probably 

 defensive adaptations against these enemies. Since the ostracoderms were 

 ancestral to later fishes, bone, rather than cartilage, must have been the 

 primitive vertebrate skeletal material, and its absence in the lampreys 



