Evolution of Animals 431 



In Cerehratidus, as described and figured by Burger, the 

 external orifice, the ciKated canal, the two diverticula, and 

 the distinct nerve branches seem well to correspond to the 

 ear orifice, the ciliated lymph duct, the sacculus and utriculus, 

 and the cochleo- vestibular nerves of the ear. That of Drepan- 

 ophorus is even more exact. Two to three auditory nerves 

 here are inserted between the two organs, the sacculus and 

 the utriculus, as in vertebrates (Fig. 16), the maculae extend 

 along their base, while the elongated process seems from 

 position, relation, and shape to represent the ductus endo- 

 lymphaticus. 



If the comparison made above be correct it follows that 

 the ductus cochlearis of vertebrates, and not the ductus endo- 

 lymphaticus, represents the primitive invagination tube of the 

 auditory organ. Further the elongated process sho\\Ti for 

 Drepanophoriis in Fig. 16 suggests exact homology with the 

 ductus endolymphaticus of vertebrates; while Burger's state- 

 ment, that it passes backward and becomes embedded in the 

 cellular tissue of the body, recalls the often extensive rami- 

 fications of it in many fresh-water teleosts. The opening 

 of its upper end on the dorsal surface of the head in elasmo- 

 branchs would then represent a special new formation. The 

 histological description of the organ given by Devoletzky 

 suggests that ciliated sensory epithelium and supporting cells 

 are here often surrounded by a mass of nerves as in the ver- 

 tebrate 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 pre- 

 sent us with suggestive stages from a comparatively simple 

 type like Carinella up to a high degree of organization. It 

 will be noted, however, that the auditory function seems largely 

 to predominate over the equilibrating one, since no distinct 

 semicircular canals are traceable. But when we remember 

 that only one exists in Myxiiie, and that two are alone formed 

 in Petromyzon, we need not wonder if the equilibrating func- 

 tion here is diffused along the sides or bases of the sac attach- 

 ments. 



The suctorial or semisuctorial nature of the mouth, along 

 the main line of animal evolution now outlined, deserves notice. 

 The swollen lips that bound the oral margin in most nemer- 

 teans have great powers of distension, so that in feeding "the 

 nemertine dilates its mouth to a large extent, and the anterior 

 end of the oesophagus is thrust out and engulphs the animal." 



