141 
Gers 
There are thirteen pairs of gills borne on the 
seventh to the nineteenth chetigerous segments inclusive 
(fig. 1). They are well supplied with blood, and, there- 
fore, generally red in colour, but in old specimens they 
usually become pigmented, and then have a dark brown 
colour. The first pair is nearly always considerably 
smaller than any of the others, and may be much 
reduced or even suppressed (fig. 10). The largest gills 
are usually found about the middle of the branchial 
region. 
The form of the gills varies considerably in the two 
varieties. In the Laminarian variety the gill consists of 
about eleven to fourteen main stems, 5 to 6 mm. long in 
full-grown worms, which radiate from a point situated 
slightly dorsal and _ posterior to the notopodium 
(fig. 19). These stems are connected at their bases by a 
web-like membrane. ‘The ventral stems are the smallest 
and apparently the last formed. Each of the main stems 
bears from ten to twelve pairs of branches, which are, 
however, not strictly opposite, but in some cases almost 
alternating. Each branch divides dichotomously a 
number of times, and may give rise to as many as twenty 
or twenty-five gill filaments (fig. 21). This type of 
gill, with its well-developed and numerous _ lateral 
branches, is known as the pinnate type. In some speci- 
mens the gills have lost some of their branches, either 
owing to friction against the sand, or to the attacks of 
enemies.* The gill of the littoral variety is not so well 
developed (fig. 22). It forms a more bushy structure, 
consisting of eight to twelve stems, about two to three 
** See, for example, the description of the attacks of the Amphipod 
Corophium longicorne upon Arenicola, M, C, d’Orbigny. Journal de 
Physique, tome 93, p. 198, 1821. 
