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 twists or coils straight to the anus, which is situated at the
junction of the trunk with the tail. A median mesentery running
dorso-ventrally supports the alimentary canal and is continued behind
it into the tail, thus dividing the body cavity into two lateral
halves.
There are no specialized circulatory, respiratory or excretory organs.
The nervous system consists of a cerebral ganglion in the head, a
conspicuous ventral ganglion in the trunk, and of lateral commissures
uniting these ganglia on each side. The whole of this system has
retained its primitive connexion with the ectoderm. The cerebral
ganglion also gives off a nerve on each side to a pair of
small-ganglia, united by a median commissure, which have sunk into and
control the muscles of the head. As in other animals there is a minute
but extensive nervous plexus, which permeates the whole body and takes
its origin from the chief ganglia. In addition to the eyes and the
olfactory circle on the head scattered tactile papillae are found on
the ectoderm.
Chaetognatha are hermaphrodite. The ovaries are attached to the side
walls of the trunk region; between them and the body wall lie the two
oviducts whose inner and anterior end is described as closed, their
outer ends opening one on each side of the anus, where the trunk joins
the tail. According to Miss N.M. Stevens the so-called oviduct acts
only as a "sperm-duct" or receptaculum seminis. The spermatozoa enter
it and pass through its walls and traverse a minute duct formed of two
accessory cells, and finally enter the ripe ovum. Temporary oviducts
are formed between the "sperm-duct" and the germinal epithelium at
each oviposition. A number of ova ripen simultaneously. The two testes
lie in the tail and are formed by lateral proliferations of the living
peritoneal cells. These break off and, lying in the coelomic fluid,
break up into spermatozoa. They pass out through short vasa deferentia
with internal ciliated funnels, sometimes an enlargement on their
course--the seminal vesicles--and a minute external pore situated on
the side of the tail.
With hardly an exception the transparent eggs are laid into the sea
and float on its surface. The development is direct and there is no
larval stage. The segmentation is complete; one side of the hollow
blastosphere invaginates and forms a gastrula. The blastopore closes,
a new mouth and a new anus subsequently arising. The archenteron gives
off two lateral pounchs and thus becomes trilobed. The middle lobe
forms the alimentary canal; it closes behind and opens to the exterior
anteriorly and so makes the mouth. The two lateral lobes contain the
coelom; each separates off in front a segment which forms the head and
presumably then divides again to form anteriorly the trunk, and
posteriorly the tail regions. An interesting feature of the
development of Chaetognaths is that, as in some insects, the cells
destined to form the reproductive organs are differentiated at a very
early period, being apparent even in the gastrula stage.
The great bulk of the group is pelagic, as the transparent nature of
all their tissues indicates. They move by flexing their bodies.
_Spadella cephaloptera_ is, however, littoral and oviposits on
seaweed, and the "Valdivia" brought home a deep-sea species.