CHORDATA. 311 



4. Cyclostomes (Lampreys). These are eeKlike animals 

 usually classed with the fishes, and are doubtless more closely 

 related to them than to the forms before mentioned, but be- 

 cause of their primitive qualities they may be placed for the 

 purposes of this course among the Protovertebrata. They 

 have a round sucking mouth destitute of jaws; they lack 

 paired appendages and the external skeleton. There is only 

 one nostril, which may or may not communicate with the 

 pharynx. The cyclostomes possess a true brain, a cartilagi- 

 nous, internal skeleton, and gills (usually 6 or 7 pairs) in 

 pouches. They differ from the true fishes in the fact that the 

 notochord is not constricted, i. e., the mesodermal sheath does 

 not, by its growth, compress it by the development of distinct 

 vertebrae around it (see 336). See figure 62. 



332. Library Exercises. By reference to all the available 

 literature make a report on the general structure, habits, and 

 important adaptations of each of the above types? How do 

 the larvae and adults of the tunicates compare? How is the 

 degeneration accounted for? To what extent is colonial life 

 represented among these types ? Are any parasitic ? Examine 

 particularly for figures of these groups in the standard refer- 

 ence zoologies. 



