Gatty Marine Laboratory, St. Andrews. 181 
The anterior hooks have the shape of a triangle, the 
posterior outline, which is slightly concave, gently curving 
to the crown and forming the base of/the triangle, the sides 
of which are formed by the anterior outline with its eight 
teeth and the peculiarly modified main fang, which resembles 
a blunt probe-like process with a small incurvation beneath, 
the prow of the hook being minute and trending witha slight 
curve to the inferior outline, which is nearly straight, and the 
body of the hook is crossed by oblique striz. The number 
of these hooks in each row is great, and in some they 
are indicated by a dark line with a dark speck at the 
ventral end. 
The posterior hooks are smaller, and have only seven 
teeth above the modified main fang, but their shape agrees 
with those of the anterior hooks. Both these and the 
anterior hooks are situated near the edges of muscular 
lamelle, a provision probably enabling them to fix on the 
walls of the tube with greater accuracy. 
A variety which occurs abundantly at North Uist, and is 
characterised by the sharp spike over the aperture of the 
tube and almost invariably a three-pronged operculum, has 
hooks which deviate from those at St. Andrews, for they 
form a long triangle and have a larger number of teeth 
above the modified main fang, viz., eight in the anterior 
hooks and nine or ten in the posterior. A similar spike 
over the aperture is met with at St. Andrews. 
Habits. The species has great vitality, surviving in impure 
water or living in a small quantity of unchanged sea-water 
for a week. One example from St. Andrews reached Perth 
on the 14th February, and lived till it was preserved on 
July 3rd in a jug of “sea-water—mean while having made a 
considerable addition to its tube, the new portion heing 
distinguished by its pure white colour. The rapid growth 
of the tube of the corresponding development of the annelid 
is sufficiently shown by the presence of well-developed 
examples on the carapace of the adult Carcinus menas. 
Orton *, indeed (1914), found that it grew to nearly full size 
in four mouths, and at this age the ova yielded practically 
100 per cent. of embryos on being artificially fertilized. 
The collar in Placostegus tridentatus, J. C. Fabricius, the 
sixth form, is so thin as to be diaphanous, but it is deep, and 
it is joined by the alar membrane dorsally on each side of 
the hiatus, passing thereafter across the ventral surface to 
* Journ. M. B. A. vol. x. p. 316, 
