PIONOSYLLIS ALTBRNOSETOSA. 239 



and a tendency to enlargement in the middle (according to the preparations), so that it is 

 somewhat fusiform, for it is narrowed at the base. It more nearly resembles the ventral 

 cirrus of Pionosyllis hyalina than that of P. prolifera. The proboscis, again, agrees in 

 structure with that in the latter form, haying ten soft papillae in front, and a single large 

 tooth. 



De St. Joseph distinguishes the form thus : " Caractere distinctif . — Soies composees 

 a serpe bidentee dans les segments anterieurs, faisant place a des soies composees a 

 serpe unidentee clans les segments posterieurs nietees a celles-ci." The examples from 

 Torquay show a general resemblance externally to Pionosyllis prolifera, whilst a scrutiny 

 of the bristles from front to rear demonstrates that the tips of the compound bristles 

 (Plate LXXXVI, fig. 13) become somewhat shorter at the eighteenth or nineteenth, seg- 

 ment and for a considerable distance thereafter, but no abrupt change is noticeable. 

 The secondary process, which is minute, has the aspect of a stouter spike terminating 

 the spinigerous edge of the terminal piece, and therefore is subject to the same influences 

 which affect the latter. The spines from the edge of the terminal piece are generally 

 absent in the middle region of the body, but in almost every foot a secondary process or 

 a trace of it can be detected in one or more bristles from the anterior to the posterior 

 region of the body, where this process is again evident. Thus in all probability the 

 normal condition is to have a secondary process or stouter spike terminating the row 

 along the edge, but with the removal of the latter, by friction or otherwise, the secondary 

 process likewise often disappears. Moreover, in a male stolon, the secondary process 

 of the compound bristles could be observed in almost every bristle in each foot from 

 front to rear, and it has to be borne in mind that the secondary process is minute and by 

 no means easily seen in any case. It would also be exceptional in the Syllicls for a 

 compound bristle with a bifid tip to lose, without external influences, the secondary 

 process after eighteen or twenty segments so as to become simply hooked, and again to 

 become bifid posteriorly. The view that friction in the intermediate region removes the 

 spines and the secondary process, therefore requires consideration, especially as the 

 bifid tips are more numerous in the middle region of the stolon. 



In comparing allied forms it is observed that in Pionosyllis hyalina the secondary 

 process of the tip of the compound bristle more resembles that in P. prolifera than in 

 P. alternosetosa. The same process is also nearer the tip in Sijllis cornuta, H. Rathke, 

 in which the tips of the bristles on the dorsal edge of the fascicle are much longer. The 

 tip in P. alternosetosa has a different character from that of P. prolifera, where the 

 secondary process is stronger, and the spinous edge seems to be less liable to injury. It 

 may be a question whether this secondary process represents the last of the spines on 

 the edge or a special development, but it has not the individuality seen in most forms. 

 The imperfection in figures renders it difficult to be precise as to the relationship stated 

 by De St. Joseph to exist between it and Syllis solida, Grrube, 1 but the latter has longer 

 dorsal cirri. 



1 ' Annul. Semper./ p. 120, Taf. vi ; fig. 7. 



