DITRYPA ARIETINA. 383 
broad fold in most Serpulids. The posterior region consists of numerous (fifty to fifty-five, 
De St. Joseph) narrow segments, most of which are devoid of bristles, and it terminates in a 
somewhat broad, flattened tail, with two rounded and sometimes prominent anal papille 
(cirri). The first segment anteriorly contains the two excretory organs, the canals of which 
unite to open between the two branchial lobes. 
The anterior bristles (Plate CX XXII, fig. 1) dilate a little above the insertion, the shaft 
then being cylindrical to the commencement of the tip, which tapers to a delicate point, 
and has narrow wings. The posterior region presents only one or two simple tapering bristles 
in each segment toward the tail. These bristles are usually slightly curved. 
The anterior hooks (Plate CX XXII, fig. la) are numerous (220, De St. Joseph) in each 
row, which runs ventrally from the bristle-tuft. The crown is small, the anterior edge covered 
with many (twenty to twenty-two, De St. Joseph) regularly arranged sharp teeth, and the 
main fang is well developed, though not sharp; whilst beneath it is a shallow gulf or notch, 
with a mere rudiment of a prow, before merging into the inferior border, which is nearly 
straight. The posterior outline presents only a trace of an incurvation, and the striz on 
the body of the hook incline from the front obliquely toward it. The entire hook is thin 
and translucent. The posterior hooks are fewer in number in each row, and considerably 
smaller, but their structure is the same. All the hooks when in position have their teeth 
directed forward. 
The tube (Plate CX XII, fig. 6a) is vitreous throughout the greater part of its thickness, 
but the inner lining is opaque white, and its shape is that of a long and sharp-pointed 
tusk of an elephant. The anterior aperture is neatly rounded, but not dilated, whilst by the 
gradual thinning of the outer layer a character is given to it. The posterior end presents 
a minute aperture. It measures from 25 to 35 mm. in length, and its wider region in front 
has a diameter of 2 to 2°3 mm. Various structures affect the external surface of the tube, 
viz., other Serpulids, such as Hydroides, Serpula vermicularis, and Spirorbis, corals (Plate 
CXXII, fig. 66), Polyzoa and sponges. It is a favourite site for Lepralie. A southern 
variety from Bono Bay is considerably smaller, the largest beme 25 mm. long, and is 
characterised by its brownish hue. In the Zetlandic examples comparatively few abnor- 
malities occur amongst hundreds. Occasionally a constriction of the shell is observed toward 
the wide anterior region, or the rings of growth here and there are unusually prominent. 
Under favourable conditions the delicate posterior end with its greater curvature is entire— 
forming a needle-like commencement to the tube. Rarely, as in certain Zetlandic examples, 
a lateral ridge on each side runs from the anterior aperture to the posterior end. 
Reproduction.—Langerhans (1880) describes the sperms as colourless, the ova as reddish. 
The earlier authors linked this form with Dentaliwm amongst the Mollusks. 
Borlase (1758) states that the tube is gently curved and represents the ‘‘case”’ of a sea- 
worm from Land’s End, but the figure gives no certainty. 
This species is not entered in the ‘Fauna Mediterranea’ of Victor Carus (1885), but it 
was dredged in abundance by the “ Porcupine” on the Algerine coast, though the specimens 
were smaller than the Zetlandic, and had a brownish tint like those procured by the 
“ Challenger ” in 50—90 fathoms off Fayal, in the Azores. 
De St. Joseph (1898) observes that Anomalina variolata, D’Orb., is fixed to several of 
the tubes. 
