THE ANATOMY OP SCALIBRKGMA INFLATUM. 279 



not related to the neuropodinm, andj indeed, if tliey are 

 connected with one or otlier division of the parapodium, their 

 relation is rather with the dorsal ramus than with the 

 ventral. The sense organs in several of the Capitellidse are 

 considerably nearer the notopodium than the neuropodium, 

 especially in the thoracic region (see, for example, Eisig, 

 1887, Taf. ii, 6g. 8 ; Taf. xxiv, fig. 3 ; and Taf. xxvii, fig. 16). 

 Tlie sense organ maybe separated from the neuropodinm by a 

 considerable interval, containing some other structure, e. g. 

 see Eisig's Taf. xvi, fig. 3, which gives a lateral view of the 

 abdomen of Dasybranchus, each of the sense organs of 

 which is situated some distance from the corresponding 

 neuropodinm, and a gill is interposed between the neuro- 

 podinm and tlie sense organ. The retractor muscle of the 

 sense organ is, in some genera, derived from the same 

 group of muscles as the protractors of the notopodial setse, as 

 in Scalibregma {see p. 273, and PI. 13, fig. 9). In these 

 cases the retractor of the " Seitenorgane " is attached to the 

 inner end of the notopodial setal sac (see Eisig's figure of 

 Notomastus lineatus, Taf. x, fig. 10). The muscles of 

 the neuropodial sette are never in any way connected with 

 the lateral sense organs. 



There is evidently, therefore, little reason for regarding 

 the lateral sense organs of Capitellids even as closely related 

 to the neuropodium, as in those cases in which the sense 

 organs are to some extent associated with one of the rami of 

 the parapodium, the association is invariably with the noto- 

 podium and not with the neuropodium. I conclude, however, 

 that the sense organ is not to be regarded as an appendage 

 of, or as intimately associated with, either ramus of the 

 parapodium; it is on neutral gi'ound between the two rami. 



I am unable to suggest any alternative to Eisig's hypothesis 

 to explain the origin of lateral sense organs in Polychaetes. It 

 may be pointed out that these organs have arisen in a well- 

 protected position near the path of a large nerve — the an- 

 nular nerve. In course of time the epidermis of this pro- 

 tected area has become much more sensitive than the less 



VOL. 45, PART 2. NEW SKRIES. U 



