PCECILOCELETUS SERPENS. 235 



organ is present (Plate CXI, fig. 11 a sp.). The dorsal bristles are long, slender, and 

 simple, but one is provided with rows of short spines directed distally. The ventral have 

 the form of three or four short, stout, slightly-curved hooks (Plate CXI, fig. 11 a), which 

 have a slight forward curve above the base, then a slight dilatation in some below the 

 forward curve of the hooked tip. They are striated longitudinally at the base, and 

 obliquely below the clear tip, which is by no means sharp, though boldly curved. The 

 dorsal division has very fine simple bristles, which are much more slender and translucent 

 than those on the first foot. 



In the third segment the cirri are smaller and more conical. The dorsal bristles are 

 long, simple, and smooth ; the ventral consist of three well-developed stout hooks, and one 

 rudimentary hook. 



The cirri of the fourth segment are somewhat less than in the third, stretch outward 

 or slightly backward, and the bristles of both divisions are now similar, viz. straight smooth 

 bristles. The fifth foot differs in having short dorsal cirri and long, slender ventral cirri 

 (the longest behind the first), and they are often arched over the back. The sixth seg- 

 ment resembles the fourth, the bristles from the third to the sixth being all smooth, and 

 thus differentiate the anterior region. The seventh commences a region extending 

 to the thirteenth, with flask-shaped cirri, which, moreover, are stiff. The bristles are of 

 two kinds, smooth, slender forms, and a few spinous bristles. Lateral sense-organs, as 

 in front, also occur, but they are broader. In segments fourteen to sixteen the feet rather 

 resemble those found in the fourth and sixth segments. The bristles are similar to those 

 just mentioned. The dorsal and ventral cirri, now conical in shape, diminish considerably 

 at the seventeenth segment, and continue of similar character to the thirtieth segment from 

 the tip of the tail, though variation in individual segments occurs. The bristles, however, 

 are remarkably developed, the slender forms of the anterior segments being replaced by 

 large, hairy, feather-like bristles (Plate CXI, fig. 11 d), the dorsal and ventral in each tuft 

 being the stronger, whilst the inner are more slender and flexible, but have much longer 

 secondary hairs, as in Plate CXI, fig. lie. The pinnate bristles also have more slender 

 shafts, the lateral hairs are longer, and Dr. Allen thought the latter were connected to the 

 shaft by a transparent membrane, but it may be mucous. These bristles aid in making 

 the current of water, which is drawn through the U-shaped tube. About the twentieth 

 foot another bristle (Plate CXI, fig. 11 e) is present, characterised by its soft or mem- 

 branous spine-like processes. The lateral sense-organs form a depression, surrounded by 

 a circular rim, the sensory hairs arising from the centre. 



The last sixteen or seventeen segments are somewhat flattened, and the dorsal 

 surface is slightly concave, and, whilst the shape is normal, the ventral cirri are about 

 double the size of the dorsal, which also are more dorsal in position. The bristles, more- 

 over, have been transformed into strong hooks (Plate CXI, fig. lib), which form a trans- 

 verse row on either side with the concavity of the hook posterior, so that they would fix 

 the annelid securely in its tube, as well as perform other functions. The other bristles in 

 these segments are either of the ordinary smooth or spiny kind ; but in the terminal region 

 (about the thirtieth segment from the end to the tenth) is a peculiar form (Plate CVII, 

 fig. 17) somewhat resembling forms seen in Eupanthalis, the terminal portion of the bristle 

 forming a kind of flexible process or brush. The lateral sense-organs are again pear- 



