68 VERTEBRATE ZOOLOGY 



the anus. The "notochord" is a very short rudimentary evagina- 

 tion of the alimentary canal. There are no pharyngeal clefts. In 

 Phoronis we approach very close to the outskirts of the phylum Anne- 

 lida; in fact it is usually classed as an aberrant annelid. In a sense 

 the Phoronidia may be considered as a link between the annelids and 

 the chordates. The propriety of classing them with the chordates is, 

 however, open to serious question. 



SUB-PHYLUM IV. CRANIATA (THE VERTEBRATES) 



All of the remaining chordates differ chiefly from those of the three 

 lower sub-phyla in possessing a cranium of some sort and a vastly 

 more complex brain than is to be found elsewhere. Although the cra- 

 niates are separated by a wide gap from the lower chordates, they form 

 within the group, especially when the extinct forms are considered, 

 a graded series from the lowest to the highest forms, which is taken 

 to indicate approximately the general course of evolution of the sub- 

 phylum. The group has been compared to the "fairly reliable and 

 complete records of a country during the historical period," while 

 the lower sub-phyla are comparable with "the few scattered and 

 scarcely decipherable documents of prehistoric epochs." 



The characteristics of the Craniata have already been outlined in 

 the first chapter in connection with the account of vertebrate mor- 

 phology. The craniates are subdivided into six classes, the various 

 groupings of which are indicated in the following table: — 



Div. 1. IcHTHYOPSiDA — ^breathing by means of gills at some period 

 in the life history; therefore essentially aquatic vertebrates. 

 They are sometimes called Anamniota {Anamnia) on ac- 

 count of the lack of an amnion, and Anallantmda on account 

 of the lack of an allantois. 

 Class I. Cyclostomata — round-mouthed fishes; lampreys 



and hag-fishes. 

 Class II. Pisces — gnathastome or jaw-mouthed fishes- 

 true fishes. 

 Class III. Amphibia — newts, salamanders, frogs and toads. 



Div. 2. Satjropsida — breathing with lungs and never developing 

 functional gills; therefore essentially terrestrial vertebrates. 

 They are, together with the Mammalia, called Amniota be- 



