24 CHORDATE ORGANIZATION n. i- 



free-swimming creature. All the other types can be derived from such 

 an ancestor, though in some cases only by what is often called 

 'degeneration'. 



2. Classification of chordates 



We may conveniently divide the Phylum Chordata into four 

 subphyla : 



Subphylum i. Hemichordata 



Balanoglossus; Cephalodiscus; Rhabdopleura 

 Subphylum 2. Cephalochordata (= Acrania) 



Branchiostoma 

 Subphylum 3. Tunicata 



Ciona, Sea-squirts 

 Subphylum 4. Vertebrata 



The Vertebrata, the largest of these groups, may be subdivided: 

 Subphylum Vertebrata 

 Superclass 1. agnatha 



Class 1. Cyclostomata. Lampreys and hag- fishes 



Class 2. *Cephalaspidomorphi. *Cephalaspis 



Class 3. # Pteraspidomorphi. *Pteraspis 



Class 4. *Anaspida. *Birkenia, *Jamoytius 



Superclass 2. gnathostomata 



Class 1. *Placodermi. *Acanthodes 



Class 2. Elasmobranchii. Dogfishes, skates, and rays 



Class 3. Actinopterygii. Bony fishes 



Class 4. Crossopterygii. Lung-fishes 



Class 5. Amphibia 



Class 6. Reptilia 



Class 7. Aves 



Class 8. Mammalia. 



3. Amphioxus, a generalized chordate 



It has long been realized that through their great variety all these 

 types show certain common features, often referred to as the typical 

 chordate characters. It is better to regard these not as a list of isolated 

 'characters' but as the signs of a certain pattern of organization that 

 is characteristic of the group. There is much reason to suppose that 

 this basic chordate organization was that of a free-swimming marine 

 animal, probably feeding by the collection of minute particles. We 

 are fortunate in having still alive a little animal, amphioxus, the 



