THE STRUCTURE AND AFFINITIES OF SACUOCIRRUS. 425 



(Fig. 48).^ Darch die lebhaftea Beweguugeu der Fiihler 

 werden in diesen Hohlraumen lose Zellen bin und her getrie- 

 ben " (p. 126 [8]). Moreover Uljanin evidently figures a 

 similar cavity in the liead of Protodrilus f lavocapitatus 

 (figs. 5, h., h'., and lb, sch. [12]). Hatschek himself^ in describ- 

 ing Protodrilus Leuckartii, mentions a cavity in the 

 peristomial segment which runs up the tentacles, and is filled 

 with reddish fluid (the blood is colourless). There seems to 

 be no doubt, then, that in Protodrilus, as in Saccocirrus, there 

 exists a special " head cavity " in connection with the ten- 

 tacles, a cavity which, in fact, 1 believe to be the modified 

 ccelom of the first segment. 



All known species of tlie genus Protodrilus are provided 

 with a ventral, muscular, pharyngeal sac below the oesophagus. 

 As further evidence in favour of the close afiBnity advocated 

 above, one may bring forward the discovery of a very similar 

 sac, lined with cuticle and with muscular walls, in Saccocirrus. 

 In the details of its structure the pharyngeal sac of Sacco- 

 cirrus seems to be more like that described in Protodrilus 

 Schneideri than like that of Protodrilus Leuckartii. 



In the presence of these facts we are forced to the conclu- 

 sion that in some important respects Protodrilus is much 

 more closely allied to Saccocirrus than to Poly- 

 gordius! Wherever we put Protodrilus in our system of 

 classification, there also we must place Saccocirrus. On the 

 other hand, that Polygordius and Protodrilus are nearly 

 related seems to be almost equally cei't'ain. There remains, 

 therefore, no alternative but to unite all three genera in one 

 group. And if we do this, many of the list of supposed 

 primitive characters of the "Archi-aunelida'^ (Polygordiidee) 

 are struck off at one blow. No longer can we enumerate as 

 characters of this group the absence of parapodia and ch^etae, 

 of external segmentation, of circular muscles, or even of 

 muscles in the wall of the gut, for these statements do not 

 apply to Saccocirrus. And, moreover, it may be added that in 

 this worm the segments are not homonomous, and the pharyn- 

 geal sac extends into the third segment. 



' I have observed a similar dorsal process of the wall iu Saccocirrus. 



VOL. 44, PART 3. — NEW SERIES. £E 



