IV 



THE VERTEBRATES WITHOUT JAWS. LAMPREYS 



1. Classification 



Phylum Chordata 



Subphylum 4. Vertebrata (= Craniata) 

 Superclass 1. Agnatha 

 Class 1. Cyclostomata 

 Order 1. Petromyzontia 



Petromyzon; Lampetra; Entosphenus; Geotria; Mordacia 

 Order 2. Myxinoidea 

 Myxine; Bdellostoma 



Class 2. *Osteostraci. Silurian-Devonian 

 *Cephalaspis; * Tremataspis 



Class 3. *Anaspida. Silurian-Devonian 

 *Birkenia; *Ja?noytius 



Class 4. *Heterostraci. Ordovician-Devonian 

 *Astraspis; *Pteraspis; *Drepanaspis 



Class 5. *Coelolepida. Silurian-Devonian. 

 *Thelodus; *Lanarkia 



Superclass 2. Gnathostomata 



2. General features of vertebrates 



All the remaining chordates are alike in possessing some form of 

 cranium and some trace of vertebrae; they make up the great sub- 

 phylum Vertebrata, also called Craniata. The organization of a verte- 

 brate is similar to that of amphioxus, but with the addition of certain 

 special features. A few of these novelties may now be surveyed, with 

 emphasis on those that provide the basis for the capacity to live in 

 difficult environments that is so characteristic of the vertebrates. 

 Firstly the front end of the nervous system is differentiated into an 

 elaborate brain, associated with special receptors, the nose, eye, 

 and ear. Through these receptors the vertebrates are able to respond 

 to more varied aspects of the environment than are any other animals. 

 Some of them have the ability to discriminate between visual shapes 

 and colours, and in the auditory field between patterns of tones, also 

 between a host of chemical substances. The motor organization 

 allows the performance of delicate movements to suit the situations 



