Evolution of Animals 445 



The primitive mode of origin of gills has been much investigated 

 and debated. In nemerteans all stages of development (Bur- 

 ger, 132-137) of two rather similar structures can be traced. 

 These have been called the "head furrows" and the "head 

 grooves"; and in some cases they seem almost to merge into 

 each other. The "furrows" usually surround the neck as a 

 pair of dorso-lateral and vertical or obhque grooves. The 

 "grooves" are rarely oblique, usually longitudinal, and run 

 forward from the neck to the front part of the head. Each 

 furrow may be a shallow or rather deep simple but ciliate 

 depression, or richly ciliated ridges may project from its cavity 

 as pointed out by Hubrecht {lJf5: 1) fully thirty years ago. 

 He then advanced the \dew that these are a rudimentary res- 

 piratory apparatus. In some genera a second pair may appear 

 in front of the normal ones. The latter or grooves are longi- 

 tudinal ciliated depressions, which in the region of the brain 

 may penetrate almost to the brain substance. Their lining 

 cells are devoid of a cuticle. Both structures are usually placed 

 close to extensive blood sinuses, as well as to the brain sub- 

 stance. But, while the furrows seem to have no nerve endings, 

 the grooves show these abundantly. 



A highly interesting and we trust exact morphological com- 

 parison can be suggested between these structures and the 

 gill-pouches of cyclos tomes. The latter have always been 

 viewed by zoologists as diverging widely from the typical 

 piscine type. But from the ciliated furrows of nemerteans 

 to the "gill pouches" of Amphibians the transition can be 

 gradually traced. Further, however, the writer would suggest 

 that the " Wimperrinnen" and " Ringf urchen" of turbellarians 

 described by Von Graff (i^^i; 2201-2204) seem in position, 

 structure, and probable function to correspond to the "head 

 grooves" and "head furrows" of nemerteans, and so to rep- 

 resent possibly the first origin of gill structures. 



The somewhat diagrammatic Figs. 19 a, b, c, d and e will aid 

 descriptions. The slight or deep paired grooves with enlarged 

 ciliated cells in various Rhabdocoela, and the paired depressions 

 of the nemerteans, with their internal projecting ciliated lamel- 



