POLYCHiETA SEDENTARIA OF THE FIRTH OF FORTH. 653 



way in which the anterior parapoclia are bent forwards, specialised in form, and 

 increased in size, for the protection of the head in other cases, 0.(7., in Trophonia, 

 Ampharete, and Pectinaria (which form a progressive series), it seems probable 

 from these examples that the operculum in Sabellaria, with its three rows of 

 paleae, is derived from the most anterior parapodia. And the examination of a 

 series of longitudinal sections of the anterior end of Sabellaria shows evidence 

 which supports this hypothesis. The cerebral ganglia are found to be placed 

 immediately above the anterior end of the oesophagus, and in contact with the 

 integument of the ventral surface of the base of the opercular peduncles. It 

 follows then that the whole substance of the peduncles belongs to the region of 

 the body behind the cerebral ganglia, that is, to the postoral somites, and not to 

 the praeoral lobe. If it were otherwise, the cerebral ganglia would of course be 

 situated on the dorsal side of the peduncles. The question next arises, Do the 

 paleae of the operculum represent one pair of parapodia, or how many pairs % 

 At present there seems to be no evidence on which to decide this question. 



The bases of the peduncles are united clorsally, but terminally they are 

 independent, and between them there is a conical process projecting from the 

 tissue which unites their bases. Each peduncle is flat towards the median 

 plane, or even slightly concave; externally it is convex. On the ventral side 

 each bears a large number of thin flexible ciliated tentacles : these are arranged 

 in a series of short transverse rows, each row being continuous at the base. 

 There are 7 or 8 of these rows. 



At the base of the opercular processes, within the cavity on their ventral 

 side, and at the dorso-lateral corners of the mouth, arise a single pair of much 

 thicker tentacles (PL XLI. fig. 17 a). These, which are also ciliated, are true 

 praestomial tentacles similar to those of Nerine. In the parts around the 

 mouth the prtestomium, the buccal somite, and the ventral part of the opercular 

 segment, must, according to our views, all be present, but they cannot be 

 defined. Behind the opercular peduncles occurs the 1st normal somite, which 

 bears the first pair of dorsal branchiae, and two chsetiferous lobes, one of 

 which is notopodium, the other neuropodium. Each of these lobes runs out 

 to a conical process anteriorly, and the ventral pair are in close relation to the 

 mouth. The chsetae are quite simple. In the next three somites, the noto- 

 podial setae are specially modified, the shape is shown in fig. 17 c 4, and it is 

 seen that they approximate to the form of the outer opercular palese. These 

 three somites, together with the preceding one, may be considered as forming 

 the prothoracic region. The succeeding somites forming the thoracic region, 

 all bear branchiae, which diminish in size towards the posterior end. The 

 notopodia are elongated fin-like processes, bearing uncini ; the neuropodial 

 setae are simple, and very long. The abdomen or tail, which is bent up 

 ventrally, bears neither branchiae nor parapodia. 



