External Characters of Arenicola 35 



conical elevation, and on or near its rounded end is the oval mouth 

 of the chaetal sac, from which the tips of a pencil of chaetae^ 

 project. The chaetal sac traverses the axis of the notopodium and 

 extends into the coelomic cavity. Fixed into the bottom of the 

 sac are the numerous capillary chitinoid chaetae, moved collectively 

 hy special protractor and retractor muscles, wliich govern the extent 

 to which the chaetae protrude. The notopodium is capable of con- 

 siderable extension and retraction ; in PI. XII, Fig. 39, it is shown 

 in a semi-contracted condition, its terminal part having been partially 

 withdrawn into the basal portion. The notopodia are of a similar 

 type throughout the genus and are present on all the chaetiferous 

 segments, except in some examples of A. hranchialis, in Avhicli the 

 notopodia of the first segment are wanting. 



The neuropodium is a muscular ridge traversed dorso-ventrally 

 by a narrow slit — the mouth of the setal sac — from whicli the tips of a 

 row of crotchets project. The section drawn (PI. XII, Fig. 39) has 

 passed immediately anterior to the chaetal sac, within which the 

 inner ends of the crotchets are indicated. The muscular ridge and 

 its hooked crotchets are excellently adapted for aiding the worm in 

 its movements up and down its burrow ; by their means one or more 

 segments can fix themselves to the wall of the burrow, and the rest 

 of the body can then be drawn towards this fixed portion by con- 

 traction of the longitudinal muscles. There are two types of neuro- 

 podia. In A. pnsilla and asdmilis the neuropodia form more or less 

 oval, cushion-like pads on the lateral portions of the chaetiferous 

 annuli ; but, even in the posterior branchial region, where the 

 neuropodia are best developed, they do not approach the mid-ventral 

 line. In the other species of the genus the neuropodia are elongate 

 muscular ridges, which, in the middle and posterior branchial 

 segments, reach almost to the mid-ventral line. The neuropodia of 

 the anterior segments (the first six or seven) are smaller than the 

 rest, and in A. cristata neuropodia are not generally visible on the 

 first two (sometimes three) chaetiferous segments. 



The posterior segments of some specimens of A. cristata and 

 loveni exhibit, between the notopodium and neuropodium, an oval 

 depression not more than -5 mm. in diameter (PI, V, Fig. 13, and 

 Fig. 44, p. 104). The position of these pits corresponds to tliat of the 



' The chaetae are not shnple hairs; they possess " Siigeblatter " (see p. 44), 

 that is, series of spine-like processes. When the chaetae are pressed into the 

 sand their processes offer resistance. These chaetae are thus more efficient 

 than simple hairs would be in aiding locomotion. 



D 2 



