SEA-FISHERIES LABORATORY. 221 



Gills. 



There are thirteen pairs of gills borne on the 

 seventh to the nineteenth chsetigerous 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 apparentlv 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 lonqicorne upon Arenicola. M. C. d'Orbigny. Journal de 

 Physique, tome 93, p. 198, 1821. 



