92 CLASSIFICATION 



duct) is directly or indirectly derived from the coelomic epithelium, 

 and may be connected in development with the pronephros. All 

 these ducts and tubules, whether urinary or genital, being coelomo- 

 stomes in origin. Provisionally one may suppose that the function 

 of carrying the ova to the exterior early became restricted to the 

 more anterior coelomostomes, belonging probably to the pronephric 

 region, and that the spermatozoa, on the other hand, passed out 

 through the coelomostomes of the mesonephric region. 



The chief characters which distinguish the Gnathostomata from 

 the Cyclostomata may be summarised as follows : There is a general 

 advance in the complexity of the various systems of organs, and in 

 histological differentiation. The primitive metamerism is more 

 completely obscured by specialisation, especially in the head region, 

 where several postauditory myotomes become reduced, and several 

 occipital segments are added from behind to the skull. The vagus 

 and some components of the hypoglossal nerve thus come to pass 

 out through the skull wall. A dermal exoskeleton arises super- 

 ficially, and the whole endoskeleton becomes much more developed ; 

 true teeth are present. The pituitary ingrowth is small, and 

 remains below the infundibulum. The nostrils and nasal capsules 

 are paired ; the ear has three semicircular canals. The first 

 visceral arch forms an upper and a lower biting jaw. Paired 

 pectoral and pelvic limbs are developed (in fish similar in structure 

 to the unpaired fins). A renal portal system occurs in the kidney. 

 A spleen is present. The genital products are carried to the 

 exterior by ducts. In the female an oviduct is developed from the 

 coelomic epithelium. The spermatozoa of the male are never shed 

 into the body-cavity, but escape through the mesonephric tubules 

 and duct to the exterior. 



The Branch Gnathostomata, including the five Classes Pisces, 

 Amphibia, Keptilia, Aves, and Mammalia, was divided by Huxley 

 [229] into the Ichthyopsida (Pisces and Amphibia), the Saurop- 

 sida (Reptilia and Aves), and the Mammalia. Since this 

 classification expresses rather the existence of three grades of 

 structure than of three divergent phylogenetic lines of develop- 

 ment, it will not here be adopted. The Reptiles must be placed 

 next to the Amphibians in any phylogenetic scheme. 



The Gnathostomes might be divided into the Ichthyomorpha 

 (Pisces) and the Tetrapoda (Amphibia, Reptilia, Aves, Mammalia). 

 But, again, there are serious objections to the isolation of the first 

 group, although the second seems genuine enough. For the Class 

 Pisces includes a heterogeneous assemblage of sub-classes, from 

 some one of which, no doubt, the Tetrapoda have been derived. 

 Therefore, until our knowledge of the internal relationships of the 



