NO. 8 THE POLYCHAET — RAW 23 



p. 4). Soderstrom, who held the latter view, sought in effect to 

 change its definition into the unsegmented anterior region of the 

 body; and he cast scorn on all who had not the same "prostomium 

 idea." Such a definition is, however, quite unjustifiable. 



The conclusion of Binard and Jeener, supported also by Gustafson 

 and accepted by the present writer, is that the primitive appendages 

 of the head comprise an anterior pair of antennae, modified in some 

 forms into palps; a second pair, never so modified; and a median 

 antenna. These are innervated by successive parts of the mid-brain. 

 The morphological order of these appendages was naturally of less 

 importance to those who regarded the prostomium as a unit, than 

 to those who thought of it as segmented. The earlier writers, previous 

 to Lameere, all placed them in their order of position — the anterior 

 pair first, the median antenna last. Lameere reversed the order, and 

 was supported by Binard and Jeener. Hanstrom seems to have 

 accepted the reversal, but did not accept Lameere's theory. Gustafson 

 was apparently in doubt, and perhaps for that reason numbered 

 them chaotically. The present theory erected in entire ignorance of 

 that of Lameere, also reverses the order, but for an entirely different 

 reason ; moreover, whereas according to Lameere the sequence of the 

 three "pairs" was already developed in the coelenterate ancestor, on 

 the present theory the three pairs of ganglia that innervated them 

 (as well as the antennae themselves) were picked at random, so to 

 speak, by nature out of a long sequence of postcephalic segments 

 during the evolution of the polychaet, and carried forward to the 

 head. Besides five antennae, the prostomium bears also two pairs 

 of eyes. Even those who have recognized segmentation in the prosto- 

 mium have not regarded the eyes as segmental ; but the author claims 

 that they belong with the intermediate antennae and the median 

 antenna, and are thus segmental in origin (pp. i6, i8, 21, and 22). 



Previous to the researches of Soderstrom on the Spionidae the 

 prostomium was always considered to include also the whole of the 

 compact mass of nervous matter constituting the brain. He, how- 

 ever, homologizing the nuchal organs with the dorsal, segmental, 

 chemical sense organs of the Spionidae, which extend through the 

 whole body, claimed that the hind-brain and nuchal organs must 

 therefore be excluded from the prostomium. On the other hand, all 

 the most primitive polychaets have a hind-brain closely associated 

 with the remainder, and possess also these nuchal organs ; hence there 

 is every reason to credit the ancestral polychaet with a hind-brain 

 and nuchal organs, and therefore to include these parts in the pro- 

 stomium. And just as the nuchal organs have their homologues on the 



