DODECACERIA. 213 



with a red mesial vessel most apparent on the anterior half. There 

 is no head, but the first ring forms a sort of cylindrical obtuse snout, 

 which is emandibulate and entirely without appendages ; the mouth 

 terminal. Eyes none. From the front margin of the following ring 

 there arise a pair of proper tentacula, and a pair of tentacular fila- 

 ments beneath them ; and a pair of these filaments come also from 

 the margin of the two or three succeeding rings. The proper ten- 

 tacula are distinguished from the filaments by being a little larger, 

 and more distinctly crenulate under the magnifier. They are all 

 nearly of the same length, filiform, smooth, varying in colour from a 

 rich brown to olive-green, but the tips are always dark. The rings 

 are distinctly defined, the eight or nine anterior nearly as broad as 

 long, forming a cylindrical portion, after which the body becomes 

 rather abruptly swollen. This enlarged portion consists of about ten 

 segments, and is followed by a hinder portion that is fully equal to 

 one-half the length of the body, and of which the rings are numerous 

 and narrow, and furnished with longer bristles. Anal segment 

 rounded, apodous, and without appendages (No. XXXVIII.). 



The bristles form two series along each side, viz. each ring has, 

 on each side, a dorsal and a ventral fascicle of them. There are 

 several bristles in each fasciqle, and of two kinds, — a long, slender, 

 flexible, setaceous kind, and the proper crochet or hooked kind. In 

 the dorsal fascicle the setaceous bristles are most numerous and con- 

 siderably elongated, intermingled with three or four hooked ones ; 

 and in the ventral fascicle the hooked bristles are a little stouter, 

 four in number, but the setaceous are comparatively few, short, and 

 weak. 



The young have only the two proper tentacula, but in other 

 respects they are like the adult. The long filaments are developed 

 in succession, and apparently not always in pairs, for I have found 

 one only in several very small individuals. The specimen from which 

 our figure was taken had six filaments besides the tentacula ; but in 

 a larger specimen there were eight, and this is the greatest number 

 I have observed*. The colour of the worm is variable, and I have 

 seen it entirely cinereous ; nor is the distinction into three portions, 

 as I have described the body, to be always perceived. Like many 

 other species of its class, it is capable of altering its form to a certain 

 extent, and never retains any one for many minutes consecutively. 



In its habits it is very interesting. It lives in a straight or slightly 

 sinuous furrow, drilled in the thickness of the shell of Cyprina 

 Islandica, — one of the most compact and hard shells of our seas. 

 How the worm bores this solid calcareous substance I am not able 

 to conjecture. There is nothing in the structure that indicates the 

 means ; and yet there cannot be a doubt that the tunnel is the worm's 

 own work. Its fitness to the body, and the unquietness and help- 

 lessness of the creature out of it, would prove this, could any one 

 who has seen the living animal entertain a doubt. When at rest 

 under water, the worm j)rotrudes the tentacula and filaments from 



* Hence Oersted's conjecture, that a difference in the number of these filaments 

 may constitute distinct species, is groundless. 



