CHAPTER XXI 



FISHES AND FISHERIES 



With this chapter we begin the discussion of the last and 

 highest branch of the animals. The branch is more commonly 

 known as the vertebrates, because all except a few of the lower 

 forms in it possess a backbone made up of a number of sepa- 

 rate vertebrae. This character separates them from all the 

 other animals that we have studied. Those forms that do 

 not have a vertebral column have, in common with the 

 vertebrate forms, in some stage of their development, apeculiar 

 structure called the notochord, which consists of a series or cord 

 of cells extending longitudinally through the body just below 

 the spinal nerve-cord. The presence of this notochord and 

 of gills in the neck region are about the only claims some of 

 the members of the branch have to be classed with the verte- 

 brates, and it is on account of the notochord that the name 

 Chordata has been given to the branch. 



The branch is divided into nine classes of which the members 

 of five are familiar while those of the other four are strange 

 small marine animals not at all popularly known. In the 

 class Adelochorda (Gr. adelos, concealed; chorde, cord) which 

 includes the worm-like Balanoglossus, the notochord is im- 

 perfectly developed and for this reason some zoologists do not 

 consider it as belonging to the Chordata. These animals 

 occur only in certain places in the sea and are of particular 

 interest only to special students or investigators. In the 

 class Urochorda (Gr. oura, tail; chorde, cord), of which the 

 ascidians, or sea-squirts, are common examples, the notochord 

 is present only in the larval stage. The ascidians when born 

 are free-swimming tadpole-like creatures with a short noto- 

 chord and a fairly well-developed nervous and digestive 

 system, eyes and auditory organs. These larvae soon attach 



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