584 DR. p. E. BEDDARD ON 



longitudinal sections it appears that the fibres constituting 

 together this muscular wall run more or less along its longer 

 axis. It will be observed that in both of these characters the 

 cirrus sac of this species differs from that of a genus recently 

 described by myself as Otidilcenia *, and, moreover, the cirrus sac 

 is very much larger in the latter genus. The cirrus itself is very 

 inconspicuous and occupies only the neck region of the sac ; it is 

 thus necessarily very short. It is distinguishable from the 

 sperm-duct by reason of the fact that it is very darkly stained 

 with both carmine and hsematOxylin. Between the cirrus and 

 the muscular walls of that part of the cirrus sac in which it lies 

 is a great accumulation of nuclei, which belong, as I presume, to 

 slender muscular fibres concerned with the retraction of the 

 cirrus. 



As in other species the cirrus sac, where it swells out into the 

 rounded flask-like body, is filled with a delicate packing tissue 

 with abundant nuclei. Through this passes the sperm-duct in 

 two or three coils. The delicate sperm-duct takes up but little 

 stain and is thus very distinct from the cirrus. I found this 

 condition of the sperm -duct to exist in a segment posterior to 

 others in which the sperm-duct had undergone even further 

 modification. In the latter segments the sperm-duct lying within 

 the cirrus sac is dilated to form a vesicula seminalis. This 

 dilated duct is also coiled ; but the two or at most three pieces 

 seen in an individual section completely fill the lumen of the 

 cirrus sac, with the exception of dividing lines filled with nuclei 

 belonging to the internal tissue of the cirrus sac. The fact 

 that an unaltered sperm-duct may lie behind one which is 

 converted into a vesicula seminalis is important. 



It is clear that, on the whole, the cirrus sac of this worm is 

 more like that of the two species which I have referred to a 

 new genus, Thysanotcenia, than it is to that of, for example, 

 Otiditcenia f or Anoplotceniat- It diflfers, however, in details 

 from the cirrus sac of both of the two species which I have 

 temporarily placed in the genus Thysanotcenia. In the con- 

 cluding part of the present communication it will be necessary 

 to go fully into the systematic position of this worm and to 

 compare it especially with the two species of the genus Thysano- 

 tcenia. It will be therefore convenient at the present moment 

 to compare the cirrus sac in these different forms. They agree 

 generally in the absence of a distal region, which I have termed 

 penis in Anoplotcenia, the cirrus being rod -like up to its free 

 extremity and not lying at the bottom of an invaginated part of 

 the cloaca genitalis. Again, in all three species the general form 

 is the same, and the muscular layer runs along the longer diameter 

 of the sac and is not specially thickened at the " neck " end. 

 These features exhaust the general resemblances between the 



* P. Z. S. 1912, p. 206, text-fig. 27. 



t P. Z.S. 1912, p. 194. 



i P. Z. S. 1911, p. 1015, text-fig. 215. 



