Dr Traquair on the Genus Gyracanthus (Agassiz). 91 



rounded by a network which is, however, line and delicate in 

 Perionyx, and appears to be completely absent in Perichceta. 

 The function of this elastic network is probably to assist in 

 extending the fibres after contraction; this would account 

 for its greater development in the outer transverse layer, 

 since, in the longitudinal layer, the contractions of the muscles 

 of one side of the body would be alone quite sufficient to 

 extend those of the opposite side. In the transverse layer 

 there is of course no such direct counteracting mechanism, 

 though no doubt the pressure of the perivisceral fluid 

 materially assists in extending the contracted fibres. The 

 elasticity of this intermuscular network must be at least an 

 additional mechanical advantage. 



This structural peculiarity of the muscular layers is not 

 found in all Oligochseta. In Urochceta there appears to be no 

 trace of it whatever. Transverse sections of the body-wall are 

 figured by Perrier in his memoir on the anatomy of the genus, 

 and show plainly that the individual muscular fibres are in 

 close contact and not isolated from each other by any septa 

 of elastic tissue ; in Pontoclrilus the circular layer of muscles 

 resembles that of TJrochmta, but the inner longitudinal muscles 

 are separated into numerous bundles by elastic septa. Finally, 

 in the genus Pleurochmta I have described a structure closely 

 similar to that of Perionyx and Perichceta. 



VI. Notes on the Genus Gyracanthus {Agassiz). By 

 Dr R H. Traquair, F.E.S. 



(Read 19th December 1883.) 

 1. Bid Gyracanthus possess dorsal sjnnes ? 



Although Agassiz himself pointed out that the spines of 

 Gyracanthus were not bilaterally symmetrical, inasmuch as 

 one side was more rounded than the other, he nevertheless 

 regarded them as dorsal, and so did people in general, until 

 in 1863 Messrs Kirkby and Atthey pointed out the probable 

 pectoral nature of some at least of these appendages, the 

 gTOunds for this conclusion being the conspicuous lateral cur- 



