Evolution of Animals 447 



As previously explained the "cerebral organ," that we have 

 viewed as a probable auditory structure, arises from some 

 part — usually the cervical or precer\dcal portion — of the groove 

 or furrow. But, from the simplest vertebrate up to man, 

 the auditory organ arises from the first gill cleft, visceral cleft, 

 spiracular cleft, or hyomandibular cleft, as it has variously 

 been called. Such a coincidence alone, between some tur- 

 bellarians, nemerteans, and vertebrates, seems more than a 

 homoplastic resemblance. It may rather afford us valuable 

 glimpses into the history of vertebrate ontogeny and organ- 

 ogeny. 



All evidence therefore indicates that these furrows or clefts 

 of nemerteans are the primitive beginnings of the first pair 

 of vertebrate gill-clefts, in which the epiblastic depression 

 often becomes quite deep, but in which also no hitherto trace- 

 able outgrowth from the pharyngeal region has arisen to meet 

 it. These probable rudimentary gills, then, are still epiblastic 

 depressions in nemerteans into which currents of water are 

 driven, but in connection with which auditory organs have 

 already arisen. 



When we consider further the abundant formation of vas- 

 cular girdles or loops, in the cervical region of some nemertean 

 genera, and their immediate proximity to the lateral furrows 

 on the one side and the oesophageal region on the other, we 

 have here the necessary fundamental requirements for for- 

 mation of at least 10-20 gill clefts with alternating arches. 

 But no such nemertean, nor type intermediate between that 

 group and the Cyclostomata, seems to exist, if we except Am- 

 phioxus, which however we would regard as an abberant side 

 type, organized somewhat along the line now desiderated. 

 But a rich series, abundant in species, may well have lived 

 during ordovician or early cambrian times, and yet have left 

 not a trace of their soft bodies in the fossil state. 



If however we may shortly try to piece together the now 

 fragmentary evidence, it seems as if, with gradual disuse of 

 the proboscis sheath, its conversion into a notochord, and 

 the establishment of currents of water into the pharyngeal 



