182
THE ESSEX NATURALIST.
although the mud was delved into to the depth of a foot—those
at Scarborough were nine inches down in the mud—and they
occurred singly, on an average of one to every ten square feet.
The worms took up a horizontal position below the superficial
brownish mud and above the deeper blue layer, and in no
instance was a specially constructed burrow seen. The rarity
of the nereid worms in the stretch of mud in which the priapuloids
were living is a feature worth mentioning, though it may be
purely accidental ; but in other places where Priapulus could
not be found the mud was riddled with the burrows of nereids
and teeming with the animals.
Priapulus caudatus is a cylindrical animal varying from one
and a half inches to two and a half inches in length and character-
ized by the possession of a curious lobed caudal appendage,
the function of which is not definitely known, though believed
to be of a respiratory nature. It ranges in colour from yellowish
to pinkish, but the tail is always a deep orange. The anterior
end—the introvert—occupies a quarter of the creature's total
body length, and is covered with conspicuous longitudinal rows
of chitinous spines, backwardly directed. At the front of the
animal a round aperture functioning as a mouth leads into a
muscular pharynx lined with very strong chitinous hooks.
Around the mouth a ring of spines larger than those in any other
part of the body, and like the others, also chitinous, stand with
inwardly directed tips like a palisade.
The pharyngeal spines and those around the mouth are not so
simple as those on the introvert. The latter are single hooks set
in small tubercles, but each of the former is comprised of seven
pointed cusps with the middle one the largest and others dimin-
ishing in size from the outer pair inwards. The hooks lining the
pharynx diminish gradually in size from the mouth to the
intestine and are arranged in a diamond-shaped pattern.
The pharynx is continuous with a straight intestine which
runs directly to the posterior end to open to the exterior. The
body wall, which is folded into a series of transverse rings, becomes
swollen posteriorly and terminates with the curious caudal
appendage, with its hollow finger-like processes.
Internally the animal has a single sac-like body-cavity in
which the internal organs lie.
The simple nervous system consists chiefly of a nerve ring