56 



Arenicolidae 



may be looked for, in the first instance on the sixteenth to nineteenth 

 segments inclusive. Subsequently gills are formed on the succeeding 

 segments, but a considerable period elapses 

 before the posterior segments acquire their 

 branchiae ; for instance, in a specimen 15 mm. 

 long, with sixty-four segments, the last twenty 

 segments are still abranchiate. 



The gills are well supplied with blood- 

 vessels and are, therefore, generally red in 

 colour, but in old specimens they become 

 pigmented, assuming a dark Ijrown colour. 

 They are sensitive and, on stimulation, 

 usually contract, their red colour disappear- 

 ing almost entirely. Specimens intended for 

 the study of gills should be narcotised before 

 being killed — successive small quantities of 

 absolute alcohol being placed on the surface 

 of the water in which they are living — so 

 that the gills may remain in an extended 

 condition. In specimens which have been 

 suddenly plunged into the killing fluid, especi- 

 ally strong alcohol, the gills are so much 

 contracted that their mode of 

 Fig. T(.—A. loveni. A, Crotchet branching is difficult to detcr- 



from the type specimen ; B, . ^ . , 



From a similar specuneii from UimC. ill OCCaSlOUal SpeCl" 



Saldanha Bay. ,, -n i i j. 



mens the gills have lost some 

 of their branches either by friction against the 

 sand or owing to the attacks of enemies, e.g. certain 

 Crustacea.^ 



• Each gill exhibits a number of main stems which 

 radiate, in the ecaudate species, from a common basal 

 trunk, or, in the caudate species, from a crescentic 

 fold, immediately behind the notopodium. In some 

 cases the crescentic fold is of considerable extent 

 and forms, for instance in A. marina, a web- 

 like membrane between the bases of the gill-axes. 

 In the other species in which it occurs, this " web " is not by any 

 means a constant character, for instance, it is present in examples 



Fig. 28. — A. ecaudata. 

 Crotchet from a 

 post -larval speci- 

 men, 8 mm. long 

 (see Fig. ^). 



^ See D'Orbigny, in Journ. Physique, xciii (1821), p. 198, for an account of 

 the attacks of the Amphipod Corojihium longicorne (now called C. volutaior) 

 on Arenicola and other worms. 



