PARAPODIA 



265 



sisting of two lobes or only one, are always well developed, and 

 project to a more or less pronounced degree from the sides of 

 the body, it is otherwise in the rest of the group, where the chae- 

 tigerous lobes are usually reduced to mere tubercles or ridges, no 

 doubt in relation to their burrowing or tubicolous habits. In 

 Sternaspis the chaetae issue directly from the body-wall. 



Amongst the Nereidiformia we find examples in which the 

 parapodia, instead of being more or less conical " legs," are 

 flattened fore and aft so as to serve as efficient " fins," as in the 

 active swimmers, Nereis virens and Nephthys caeca, and in the 

 pelagic Phyllodocids, Alciopids, Typhloscolecids, and Tomopteris. 



Fig. 137. Parapodia. 

 A, Polyuoe ; B, Sco- 

 loplos; C, Euphrosyne. 

 (Transverse section of 

 body.) a', Accessory 

 cirrus ; y, doubtful 

 branchiae : D, SdbtUa 

 (thoracic). a, Noto- 

 podial cirrus (' : ely- 

 tron " in A, " gill " in 

 B) ; b, notopodiuni ; 

 c, neuropodium ; d, 

 neuropodial cirrus ; n, 

 aciculum (accidentally 

 omitted in C). 



C D 



Of the typical dorsal and ventral cirri, the ventral is only- 

 absent in some Amphinomids amongst the Nereidiformia ; the 

 dorsal is absent in Nephthys and degenerate in Glycera, whilst 

 in a very large number of families of the other sub -Orders 

 neither cirrus is present. These cirri, though originally fila- 

 mentous and sensory, may, by virtue of special blood supply, 

 become " gills," and this occurs in several families of different 

 sub-Orders. Thus in Eunice this gill is comb-like; in Ampki- 

 nome and in Arenicola (on certain segments) it is arborescent, 

 as it is also in one to three segments in Terebellids ; whilst in 

 Arieiidae, Spioniformia, Cirratulidae, Opheliidae, and Sabdtaria 

 it remains more or less finger-shaped or filamentous. In the 

 family Serpulidae the thoracic cirri, both dorsal and ventral, 



