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 

 pigmeuted, assuming a dark "brown 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. 27. A. looeni. A, Crotchet branching is difficult to deter- 



from the type specimen ; B, , ' 



From a similar specimen from mine. In OCCaSlOlial SpeCl- 



Saldahha Bay. ,, .,, , , 



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. 1 



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 cresceutic 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 



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

 the attacks of the Amphipod Coropliium longicorne (now called C. volutator) 

 on Arenicola and other worms. 



Fig. 28. A. ecaudata. 

 Crotchet from a 

 post - larval speci- 

 men, 8 mm. long 

 (see Fig. 5). 



