366 POMATOCERUS TRIQUETER. 
are short, and slightly tapered from base to apex. The branchiee in young forms are pale, 
the central part of the middle region reddish, and the posterior part pale cream-colour. 
A white band of great purity runs round the base of the branchie, bordered by a belt of 
red on each side. The filaments are barred alternately with white and red. Methylated 
spirit alters the red colour of the branchial plumes to a permanent blue. De St. Joseph 
found Trichodina pediculus, Klw., on the branchiz at Dinard, which he has made classical 
to all marine zoologists. 
The single operculum (Plate CX XII, fig. 2a) springs from the dorsal edge of the left fan 
by a somewhat rounded stem, enlarges and flattens out as it proceeds upward, and dilates 
at the summit into a large flat cushion for the support of the calcareous operculum, which 
is prettily marked with white (shield-hke). Just before the last-mentioned dilatation it 
sends out a process on each side, and occasionally this auricular process is bifid, or quadrifid. 
The calcareous tip assumes various forms, viz., conical, bluntly tuberculated, saucer-shaped, 
bifid or very often trifid. On removing the calcareous cap, a tough layer is left on the 
summit of the cushion with projections corresponding to the form of the cap. The cone 
is not regular, but has its steepest side placed dorsally. Aiter the action of hydrochloric 
acid on the operculum a brownish scale of organic tissue with somewhat regular hexagonal 
reticulations is left. Internally the opercular pedicle has a strong elastic ligament 
from the summit to the base, surrounded by muscular bundles—chiefly transverse. The 
thrusting out of the operculum (if such happens) is thus voluntary, the withdrawal and 
retention more or less involuntary. 
The cuticle of the entire opercular apparatus is tough and glistening, faintly marked 
under the microscope with fine striz. The colour of the operculum varies, most being pale, 
the stalk often presenting two specks of brown. Almost every example from Lochmaddy, 
North Uist, has an operculum with three prongs, the only exception being one here and there 
with only two prongs, but in all probability a third would by-and-by appear. A variety 
with a flat operculum from which the three spikes arose was also occasionaily met with. 
The operculum is a favourite site for parasitic growths such as the chambered Fora- 
minifera, Vorticella, and zoophytes. 
In minute structure the body-wall anteriorly shows a thick coat dorsally of long, narrow, 
hypodermic cells, so that such might be supposed to act as a substitute for the chordoid » 
skeleton of other forms. The projection of the opercular stalk causes asymmetry of the 
body-wall, and of the incipient dorsal longitudinal muscles, for the muscle of the opercular 
side considerably increases in size, and this asymmetry continues after the stalk separates. 
It disappears when the branchial filaments approach separation. 
In the thoracic region the dorsal and ventral blood-vessels are of large size, and the 
rete around the alimentary canal is well developed, forming a belt of longitudinal vessels 
as observed in the section. 
When the body-wall assumes a more symmetrical outline, the special, thin, longitudinal 
muscular bands on each side have disappeared, and the actual longitudinal ventral muscles 
are formed on the lateral region outside the anterior glandular organ, and its appendix 
toward the tail. Again, the dorsal longitudinal muscles are connate in the middle line 
and occupy almost two-thirds of the circumference.! 
' Vide further details in ‘Ann. Nat. Hist.,’ ser. 9, vol. ii, pp. 41—55. 
