314 ZOOLOGY 



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 vertebrae 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. 



