28 GONADS 



Craniata., excepting the Cyclostomes. In the female sex are 

 generally found opening into the eoelom a pair of oviducts, the 

 homology of which is still uncertain. They appear to be as a rule 

 connected with, if not derived from, the pronephric tubules in 

 development (p. 89). 



As for the gonads themselves, they are originally paired organs 

 extending along the dorsal wall of the abdominal eoelom into 

 which they hang, and from the epithelium of which they develop. 

 They form, in the embryo, two longitudinal genital ridges situated 

 near the base of the mesentery ; when the gonad is single in the 

 adult, this is due to the fusion of the tAvo rudiments, or to the 

 suppression of one. Only doubtful traces of metamerism have been 

 described in the gonads of the Craniata. 



From what has been mentioned above it is obvious that many 

 important characters, such as the possession of the paired sense- 

 organs, the extreme cephalisation of the head segments, the 

 structure of the skeleton, brain, heart, liver, kidneys, and gonads, 

 distinguish the Craniata from the Cephalochordr. They appear 

 to have been fully developed in the early common ancestor of the 

 Craniata (there is perhaps some doubt about the paired olfactory 

 organ, see p. 39), and these characters clearly demonstrate that the 

 Craniata must have travelled a long way from their common 

 starting-point with the Cephalochorda, and must have passed 

 through a long series of intermediate forms of which we have 

 now no trace, before they began to diverge into the various groups 

 included in our modern classifications of the subphylum. 



Two diverging branches from the Craniate stock are repre- 

 sented at the present day, the Cyclostomata and the Gnathostomata. 



