y.
e, Ovary.
el, Oviduct.
ho, Testes.
sg, Vas deferens.
f^2, f^3, Lateral and caudal fins.
sb, Seminal pouch.

The eyes are indicated as black dots behind the cerebral ganglia.]

As a rule the body is some 1 to 2 or 3 cm. in length, though some
species are larger, by 4 or 5 mm. in breadth, and it is shaped
something like a torpedo with side flanges and a slightly swollen,
rounded head. It can be divided into three regions--(i.) head, (ii.)
trunk, and (iii.) tail, separated from one another by two transverse
septa. The almost spherical head is covered by a hood which can be
retracted; it bears upon its side a number of sickle-shaped, chitinous
hooks and one or more short rows of low spines--both of these features
are used in characterizing the various species. A pair of eyes lie
dorsally and behind them is a closed circlet, often pulled out into
various shapes, of modified epidermis, to which an olfactory function
has been attributed. The interior of the head is filled up with masses
of muscle fibres which are mainly occupied with moving the
sickle-shaped hooks. The trunk contains a spacious body-cavity filled
during the breeding season by the swollen ovaries, and the same is
true of the tail if we substitute testes for ovaries.

The skin consists of a transparent cuticle excreted by the underlying
ectoderm, the cells of which though usually one-layered may be heaped
up into several layers in the head; beneath this is a basement
membrane, and then a layer of longitudinal muscle fibres which are
limited inside by a layer of peritoneal cells. The muscles are
striated and arranged in four quadrants, two dorso-lateral and two
ventro-lateral, an arrangement which recalls that of the Nematoda,
whilst in their histology they somewhat resemble the muscles of the
Oligochaeta. Along each side of the body stretches a horizontal fin
and a similar flange surrounds the tail. Into these fins, which are
largely cuticular and strengthened by radiating bars, a single layer
of ectoderm cells projects.

The mouth, a longitudinal slit, opens on to the ventral surface of the
head. It leads into a straight alimentary canal whose walls consist of
a layer of ciliated cells ensheathed in a thin layer of peritoneal
cells. There is no armature, and no glands, and the whole tract can
only be divided into an oesophagus and an intestine. The latter runs
with no t