NO. 8 THE POLYCHAET — RAW II 



and the visceral nervous system are closely associated with one 

 another. Throughout their common extension the visceral nerves 

 and ganglia are in the stomodeal hypodermis. And it is suggested that 

 these two systems are also connected in origin ; indeed, that the 

 visceral nervous system is a direct result of the stomodeal invagina- 

 tion; that before the invagination commenced, the present visceral 

 nervous system would not exist, and the brain would consist only of 

 fore-brain ! 



The stoinodeutn is very complex and the invagination that gave 

 rise to it must have been very complex, probably much more so 

 than we can realize (see Heider, 1925, figs. 5 to 10; also figs, i and 

 2 in this paper, p. 6). Though so complex, it divides itself quite 

 naturally into three divisions: (i) the oesophagus; (2) the sacs 

 associated with the upper jaw armature; and (3) the sacs associated 

 with the lower jaw armature. Three successive invaginations or 

 series of invaginations appear therefore to be represented, which 

 must have been separated by long halts, and probably imply changes 

 of feeding. The first is represented by the oesophagus; and this 

 would seem to have very long antedated the other two, represented 

 by the upper jaw series and the lower jaw series of pharyngeal sacs, 

 which were separated by a shorter interval. At each of these later 

 invaginations, the new was not merely an extension of the old, but 

 a new structure lying ventrad of the old (fig. i). 



The first or oesophageal invagination was probably a protracted 

 process; it is represented by the long oesophagus, commencing in 

 front of the mid-gut and bounded by the lips of the pharyngo- 

 oesophageal rift (/ I, figs, i and 2), which, but without the rift, may 

 represent the lower lip at the end of that stage. 



The second or upper jaw series was also probably a long process : 

 its effects were to produce the main cavity of the pharyngeal sac 

 with the upper jaw sac above it, and bounded below by the "fore- 

 pad," which again, but without its rift, may represent the lower lip 

 of that stage. It contains the so-called "upper jaw" apparatus, the 

 different elements of which — the bearers, fangs, saw-plates, and 

 "rub plates" — are here interpreted as representing several pairs of 

 appendages! In the more primitive eunicid, Cirrohranchia parthe- 

 nopeia, seven such pairs of appendages might seem to be indicated ; 

 but comparisons throughout the suborder show that it is impossible 

 to judge of the number involved, and this is not surprising when it 

 is realized that the evolution of the stomodeum took place very long 

 before Cambrian time and probably over 1,000 million years ago. 



The third or lower jaw invagination adds a further pair of pockets 



