24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



postcephalic segments of the Spionidae, so the antennae have theirs 

 in the dorsal cirri of the body segments, and the eyes have theirs in 

 the postcephaHc eye-spots. The principle on which Soderstrom would 

 exclude the hind-brain is invalid, and is reducible to an absurdity. 

 On the theory here advanced it would also exclude the whole of the 

 mid-brain, the antennae, and the eyes. Furthermore, reason has 

 above been given for the incorporation of the hind-brain at the same 

 time as mid-brain III. Again, on the theory advanced by Hanstrom 

 it would exclude the fore-brain and the mouth-lips! The principle 

 is wrong: Soderstrom did not allow for any evolution within the 

 annelid. We must get back to the primitive idea, and define the 

 prostomium as all that region in front of the mouth inherited in that 

 position from the primitive polychaet. On the theory here advanced 

 this includes : (i) the fore-brain and the representative of the "mouth- 

 lips," inherited from the head of the more primitive annelid, to- 

 gether with the following parts acquired in order by the prostomium 

 during the evolution of the head of the polychaet: (2) the hind sec- 

 tion of the mid-brain (mid-brain I) with the median antenna (due 

 to coalescence of a pair), and a pair of eyes (the posterior pair) ; 

 and long afterward (3) the middle section of the mid-brain (mid- 

 brain II) with the second pair of antennae, and the anterior pair of 

 eyes ; and again long afterward (4) the anterior section of the mid- 

 brain (mid-brain III) with another pair, the anterior, of antennae; 

 and also (5) the hind-brain with the nuchal organs; these two brain 

 parts (mid-brain III and the hind-brain) being probably acquired 

 nearer the same time. 



Gustafson, who is doubtful whether to accept the theory of 

 Hanstrom that the stomatogastric lobes (or fore-brain) have been 

 added from the ventral chain, is doubtful therefore whether to regard 

 the mouth-lips, which they innervate, as part of the prostomium. 

 Nothing better illustrates the enormous contrast between the present 

 theory and that. What Hanstrom regards as the last addition is here 

 claimed as the original nucleus. 



Many may be surprised at the inclusion here of the hind-brain 

 after Soderstrom's important and significant work; but it will be 

 clear that the same principle, which would exclude it, would exclude 

 also the whole of the mid-brain and all the cephalic tentacles and eyes. 



It will thus be seen that the theory here advanced presents a new 

 conception of the prostomium. It is not the representative in modern 

 forms of the head of the primitive annelid, but the result of the long 

 evolution from this of the head of the polychaet. It is not a unit as 

 so many have supposed, but an aggregate, acquired only in the 



