Evolution of Animals 429 



it is supplied by two olfactory nerves from the brain, it might 

 equally be suggested that two of the three areas in Cerebrahdus 

 have temporarily fused, or that a single original olfactory 

 nerve is splitting, preliminary to separation of the olfactory 

 organ into two structures seen in other vertebrates. Minute 

 comparative study will alone determine the question. But 

 from the studies of Brookover (i-4^.*49) and of C. J. Herrick 

 (162: 413) it seems highly probable that one median mass in 

 front of the brain originally constituted the olfactory body. 

 Di\dsion of the sensory olfactoiy sac into halves, as seen pro- 

 ceeding in transition from Myxine to Petromyzon, has evi- 

 dently resulted in splitting of the mass into halves, so that 

 a primitive common "placode" is diihcultly traceable. 



In the Nemertinea eyes may either be absent (by absorp- 

 tion?), or they consist of a simple pigment spot, or show ad- 

 vancing stages up to an organ with retinal, choroid, and lens 

 constituents. That they vary in number from 20 down to 

 2, but tend in most cases to be paired on either side of the 

 head, suggests that we are here dealing with diffuse evolving 

 sense centers for heliotropic perception such as still occur in 

 many Turbellaria, but that become reduced to two paired 

 highly developed organs in vertebrates, and to a median pari- 

 etal pair still largely functional in Petromyzon and lizards, and 

 probably retained along the evolving vertebrate line, though 

 functionally absorbed before the mammals had developed. 



The "cerebral organs" of nemerteans have been much dis- 

 cussed, though unanimity of opinion as to their function has 

 not yet been reached. These however seem, like most other 

 parts of nemerteans, to be the simple rudiments of more evolved 

 structures in vertebrates. They are paired organs developed 

 along the postero-lateral sides of the head. Each in its sim- 

 plest state is "a mere groove in the epidermis not extending 

 deeper than the basement membrane; it is lined by ciliated 

 cells, and at the bottom are large gland cells; while the organ 

 is supplied by nerves from the dorsal ganghon of the brain. 

 In Carinella ruhicunda and others the groove becomes an 

 oblique canal, the blind end of which is surrounded by a mass 

 of ganglion cells, lying outside the cutis. In the higher forms 

 the canal penetrates deeper into the body as far as the brain. 

 The gland cells and the associated nerve tissue increase in 

 amount, and the canal becomes differentiated into two regions 

 — an extra-ganglionic 'lateral canal' and an intra-ganglionic 

 'cerebral canal' (c) which frequently terminates in an enlarged 

 sac. In Drepanophorus the cerebral canal is quite exceptional, 

 in that it bifurcates, one branch terminating in a sac with sen- 



