226 MAGELONA PAPILLICORNIS. 



series of delicate flattened bristles, each with an expanded translucent tip rounded at the 

 edges (Plate CVI, figs. 7 b, 7 c, 7 d 9 and 7 e), and distally with a short prong. The shaft 

 of the bristle is marked by a line which disappears at the enlarged end near which the 

 shaft is curved. This line or ridge may indicate modification of a winged bristle, of 

 which the rounded tip is an expansion, and various degrees of development may be 

 traced. Thus the dilatation at the tip is comparatively slight in those (Plate CVI, 

 fig. 7 b) next the few simple bristles ; then a larger tip is found (Plate CVI, fig. 7 c), 

 followed by the well-marked series (Plate CVI, figs. 7 d and 7 e) in which the shaft tapers 

 to a fine point near the tip, whilst the broad wings continue and coalesce into a trans- 

 lucent expansion with the terminal filiform process. In the dorsal group there is only a 

 tendency to the formation of the simple bristles at each end of the row, the rest leading 

 to those with the spatulate translucent tips. Amongst them are some longer forms, also 

 translucent, each having the aspect of a shortened and widened capillary bristle, with 

 double translucent wing, a double curve, and a tapered tip, but less tapered and broader 

 than the tips of the anterior series. Intermediate forms also occur with a somewhat 

 widened tip and a longer central axis or prong. 



The two dorsal lamellae approach each other closely in the middle line, thus differing 

 from the feet of the anterior and posterior regions. 



The ventral division of the ninth foot forms a transversely elongated fan-like lamella 

 stretching inward across the body. It bears the same remarkable bristles, and, in 

 addition, a group of about three long, slender capillary bristles with traces of wings at 

 the tip. These have not been observed by Mesnil. The two ventral lamellae are 

 separated by a considerably larger median gap than the dorsal. The segment to which 

 these bristles belong seems to have atrophied, that is to say, it is represented only by the 

 attached surface of the lamellae. 



The ninth segment indicates a distinct region, and its bristles would appear to be 

 homologous with the special forms in the third segment of Disoma, the fourth in the 

 Chaetopteridae, and the fifth in Polydora, and probably are of service in protruding the 

 anterior region from the sand into the surrounding water for aeration and food, as well 

 as forming a special fulcrum during the movements of the snout and the anterior region 

 in boring. 



The tenth foot presents two broadly ovate lateral lamellae with narrower attach- 

 ments, which are situate between the processes bearing the winged hooks. 1 The dorsal 

 process forms a thin lamella broader at its outer margin and running to the surface of the 

 body at its inner edge. It carries about a dozen hooks, with the points of the main fang 

 directed outward (Plate CVI, figs. 7f, 7 g, and 7 h). The ventral process has much less 

 depth and is shorter, and soon reaches the surface internally, a small conical cirrus 

 marking its inner edge. The position of the respective parts is thus in keeping with 

 their functions. The prominence of the winged hooks dorsally and ventrally places them 

 in ready touch with the wall of the sand-tunnel, whilst the delicate lamellae can be folded 

 flatly against the lateral wall between them. De St. Joseph states that the tenth 

 segment and the following are biannulate, the posterior one bearing the feet. 



1 Mesnil observes that the external part of the winged hook in this form and in the young 

 Nerine cirratultis is paired, but what is probably meant is that the wing is double. 



