. Catalogue of the Chaetopoda in the British Museum (Natural History). Oligochaeta; Polychaeta. Crotchets of Arenicola 51 A. pudlla, Fig. 24). A crotchet from a specimen of A. assimilis var. affinis, 208 mm. long, is drawn in Fig. 22 b. It is 1 • 1 mm. in length, and its characters—the elongate edentulous rostrum (a little worn at the tip) making a wide angle with the shaft—are such as would be expected in a crotchet from so large a specimen. A. glacialis.—The crotchets of this species are similar in general form to those of A. marina, and have no post-rostral dilatation (Fig. 23). A neuropodi
. Catalogue of the Chaetopoda in the British Museum (Natural History). Oligochaeta; Polychaeta. Crotchets of Arenicola 51 A. pudlla, Fig. 24). A crotchet from a specimen of A. assimilis var. affinis, 208 mm. long, is drawn in Fig. 22 b. It is 1 • 1 mm. in length, and its characters—the elongate edentulous rostrum (a little worn at the tip) making a wide angle with the shaft—are such as would be expected in a crotchet from so large a specimen. A. glacialis.—The crotchets of this species are similar in general form to those of A. marina, and have no post-rostral dilatation (Fig. 23). A neuropodium of one of the fragmentary type specimens yielded large crotchets of the type shown in Fig. 23 c, in which the rostrum is longer and is curved almost like the blade of a scythe. This represents probably the final growth-phase. —The crotchets of this species exhibit a full rounded curvature of the region behind the rostrum, so that the free end of the crotchet resembles the head and beak of a swan. This character is shown best in small or medium sized specimens (Fig. 24 A, b), and is especially marked in the crotchets of the type specimen (Fig. 24 b). It is also obvious, but to a less degree, in the large crotchets (Fig. 24 c), which represent the latest growth-phase seen by the writer, of a massive specimen 160 mm. in length. The head-like curvature is more strongly marked in this species than in A. cristata. As A. pusilla is one of the most difficult species to identify, this character is of consideralile value, for in the two species with which A. picsilla is most easily confused, namely, A. marina and assimilis var. affinis, the crotchets do not present any dilatation of the post-rostral portion. A. cristata.—The sequence of growth-phases (Figs. 25, 26) has been found to be similar to that described for A. marina. In crotchets (Fig. 26 d, e), which show the final phase of growth, from E 2. Fig. 22.—A. agximills var. ainniit. Crotchet from a specimen
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