28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



So close are these comparisons that one can have little doubt but 

 that all derive from the same ancestral stock; and if the fundamental 

 plan of the brain above detailed be accepted, it will be agreed that 

 the closest approach to this ancestral stock is to be found in the 

 Eunicimorpha. The evolution which has been traced, if it be sub- 

 stantially true, is therefore that of the ancestral polychaet. It can 

 all be assigned to very far back in pre-Cambrian time, effected say 

 before i,ooo million years ago. Of this the writer feels confident; 

 because, as he hopes to show, the arthropods, some of the classes of 

 which were evolved long before Cambrian time, were derived from a 

 particular family of polychaet still extant ! 



If the above given conclusions as to the character and origin of 

 the original polychaet brain be accepted, its further evolution can 

 with some confidence be pictured. The separate ganglia of the two 

 sides of the brain united in the midline, consolidation was effected, 

 and enormous development has ensued — development which can be 

 read by detailed comparisons of the brains of the various polychaets. 

 The fore-brain, hitherto dominant, has lost its old eminence even in 

 the Eunicidae; and in other families is often greatly reduced. The 

 mid-brain has undergone great development, and perhaps in all forms 

 has become the chief nervous center. And whereas, as is here 

 assumed, the palpal function in the primitive polychaet was seated 

 in the mouth-lips, and served by the fore-brain, now only in the 

 Eunicidae and Amphinomidae is this the case. In some others it is 

 apparently seated in the proboscis and is still served by the fore-brain : 

 in yet others (viz, the Aphroditidae, Chrysopetalidae, Hesinoidae, 

 Syllidae, and Nereidae) the function has been transferred to the 

 anterior pair of antennae, transformed into palps, and is served 

 mainly by the mid-brain. This is indeed the case even in a family 

 of the Eunicimorpha — the Stauronereidae (Staurocephalidae). The 

 degrees of development of the brain are extremely varied ; and the 

 new structures include neurone courses, nerves, commissures (trans- 

 verse in considerable number, and also longitudinal), the true brain 

 ganglia, and in the highest forms the corpora pedunculata. In the 

 Amphinomorpha extra pairs of nuchal ganglia have joined the hind- 

 brain and this has here become much the most voluminous division 

 of the brain. With different modes of life very different lines of 

 evolution were followed. All other suborders seem to have changed 

 more than did the eunicid. Here the brain is still relatively primitive, 

 exhibiting its primitive subdivisions still extended in a plane. In 

 contrast, in the aphroditid it is compact, folded over between front 

 and rear and very highly developed. On the other hand, in some 



