Evolution of Fishes from Invertebrates 85 



teleosts. The opening of its upper end on the dorsal surface 

 of the head in elasmobranchs would then represent a special 

 new formation. The histological description of the organ 

 given by Devoletzky suggests that ciliated sensory epithe- 

 lium and supporting cells are here often surrounded bv a 

 mass of nerves as in the vertebrate ear; while the secretion 

 of mucus from surrounding gland cells and the presence 

 often of refractive 'kornchen' — that are possible rudiments 

 of otoliths — is most instructive." 



"A minute comparative study of living nemerteans may 

 yet reveal more exact relationship, while the group seems 

 to present us with suggestive stages from a comparatively 

 simple type like Carinella up to a high degree of organi- 

 zation. It will be noted, however, that the auditory func- 

 tion seems largely to predominate over the equilibrating 

 one, since no distinct semicircular canals are traceable. 

 But when we remember that only one exists in Myxine, 

 and that two are alone formed in Petromyzon, we need 

 not wonder if. the equilibrating function here is diffused 

 along the sides or bases of the sac attachments." 



XI. The blood-vascular system. 



Some authors have held that the primitive vascular sys- 

 tem in vertebrates consisted primarily of a dorsal and a ven- 

 tral vessel. Thus Bridge {36: ^^A-) says: "There is little 

 doubt that, primarily, the vascular system of vertebrate 

 animals consisted of a dorsal artery (dorsal aorta), running 

 along the median dorsal line of the alimentary canal, and a 

 ventral or subintestinal vein similarly related to the ven- 

 tral surface of the digestive tube. The two vessels were con- 

 nected by a series of pairs of lateral branches, which had 

 their origins from the dorsal vessel, and, by their subdivi- 

 sion, formed a capillary network in the walls of the aliment- 

 ary canal. From these networks paired veins issued and 

 opened into the subintestinal vein." We can now compare 

 the above with the system in meta- and hetero-nemerteans. 

 In these the main vascular system consists of a longitudinal 

 dorsal vessel that runs in line between the proboscis sheath 

 and the alimentary canal, also of two lateral paired trunks 



