CHORDATA 315 



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 cartilaginous 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 develop- 

 ment of distinct vertebras around it (see §344). See Fig. 64. 



340. 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. 



