. Catalogue of the Chaetopoda in the British Museum (Natural History). Oligochaeta; Polychaeta. 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 pigmente
. Catalogue of the Chaetopoda in the British Museum (Natural History). Oligochaeta; Polychaeta. 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, 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. Please note
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