xm 



PHYLUM CHORDATA 



249 



defined rounded mass, or yolk-^sac, to be subsequently absorbed as 

 development proceeds. The most important features in the later 



FIG. 923. Ceratodus forsteri. Stages in the development. A, lens-shaped blastula ; B t 

 stage with semicircular blastopore (bl. p.) ; C, later stages in which the blastopore (bl. p.) 

 has taken the form of a ring-like groove enclosing the yolk-plug (ylk. pi.) ; D, stage in which 

 the narrow medullary groove (blp. sut.) has appeared with the rudiment of the medullary 

 folds (med.) ; E, stage in which the medullary folds (med.) have become well developed ; F, 

 later stage with well-formed head with two visceral arches (vise.) and rudiments of eye 

 (eye) and ear (aud.) ; pron. mesonephros. (After Semon.) 



stages (F) are the negative ones of the absence of the external gills 

 (to be referred to below and in the account of the Amphibia), and 

 the absence of horny jaws. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Dipnoi are Pisces in which the notochord is persistent, 

 there are no vertebral centra, and the primary cranium persists 

 with little ossification, but has added to it a number of investing 

 bones. The skull is autostylic, the lower jaw articulating with a 

 palatoquadrate process which is immovably fixed to each side of 

 the skull. There are four to six cartilaginous branchial arches. 

 The dermal fin-rays are slender more or less ossified fibres, and are 

 supported by numerous cartilaginous or ossified pterygiophores. 

 The caudal fin is diphycercal. The paired fins are of the character 

 of " archipterygia." The pectoral arch is a single cartilage with a 

 pair of superficial investing bones on each side. The pelvic arch is 

 well-developed and cartilaginous. There are gills attached to the 

 branchial arches, and in addition a single or double lung opening 

 into the oesophagus by a ventral aperture. The gills are covered 

 over by an operculum. There is a dermal skeleton in the form of 



