BALANOGLOSSUS, the name given by the Neapolitan naturalist, Delle Chiaje, to certain fragmentary specimens of a marine creature brought to him by a fisherman about 182o. The creature would be termed by any ordinary person a "worm" as it is soft bodied, without legs or other appendages and burrows in sand or mud at the bottom of comparatively shallow sea. The body is somewhat flattened and produced into a lobe or flap on each side and hence Delle Chiaje thought it looked like a tongue; but in front there is a pointed salient portion issuing from a collar-like ring, and this reminded Delle Chiaje of a barnacle (q.v.), hence the name, which literally means "barnacle-tongue." Subsequently researchers into antiquarian zoology discovered that four years previous to the publication of Delle Chiaje's work in 5829 a Russian naturalist, Eschscholtz, had described a similar "worm" from one of the Pacific islands and had called it Ptycho dera. Eschscholtz had mistaken the "worm" for a sea cucumber and his discovery attracted no notice at the time and was for gotten. Delle Chiaje's description however awakened interest and drew the attention of many other observers; the "worm" proved to have many startling features in its anatomy. The name Balan oglossus had thus become firmly embedded in zoological literature when it was stated Ptychodera must be used instead.
In time "worms" like these were found all over the world in the warm and temperate zones, wherever suitable conditions of muddy sand existed ; they differed in the proportions of the various parts of the body. Delle Chiaje's old genus eventually became an order.
The difficulty created by the historiographers was then partially got over by using both the names Balanoglossus and Ptychodera
for two of the genera into which the old genus was divided.
The name Balanoglossus has been
retained also as a popular
lation for the whole group and
in that sense it will be used in
this article.
vided into three regions, as shown
in fig. 1, which represents
scholtz's Ptychodera. In front is
the conical projection now
termed the proboscis and which
was Delle Chiaje's barnacle.
hind it comes the "collar" region
and behind this the long trunk. In
the genus Ptychodera this is
duced at the sides into two great
flaps, but in Balanoglossus (sensu
stricto) these are absent. In every
species however along the upper
surface of the trunk in front
there are found two long rows of
pores which penetrate right
through the alimentary canal.
Nothing like these is known
where else in the animal
dom except the gill-slits of fish
and other aquatic vertebrates.
Balanoglossus resembles in its
food and mode of life many
rowing creatures of widely
similar structure, amongst others
the common lugworm and earthworm. Feeding and movement are
essentially the same operation. As it burrows, it swallows the
muddy sand and extracts the organic matter. Its mode of motion,
however, is peculiar ; the proboscis possesses a single pore and the
collar a pair of ciliated pores. By these water is sucked in and
the cavities of these regions distended. The muscles of these
stiffened parts then wriggle and drag the passive trunk after them.
The reproductive organs are mere groups of cells situated in
the sides of the trunk—in the flaps when these are present. The
cell masses when ripe burst to the exterior, making their own
openings—just as boils do in the human body. The sexual cells
are discharged into the sea, their union being left to chance. From
the fertilized egg there is developed an extraordinary larva, the
tornaria, which strikingly resembles the larva of an Echinoderm
(q.v.) and indeed was mistaken for such by its discoverer
hannes Muller). This larva (fig. 2) is of glassy clearness. It
possesses a gut consisting of gullet, stomach and intestine. The
anus is at the posterior end of the body but the mouth lies on
the under surface far behind the front end. Its movement is
made by a peculiar folded ridge of skin which carries long cilia.
This ridge is shaped roughly like an H, the sides of which run
from anterior to posterior ends of the larva ; whilst on the central
surface deeply enfolded cross-pieces run backwards on the lobe
in front of the mouth and forwards in front of the anus. In
addition there is an independent ring of cilia round the hinder end.
In front of the mouth there is a thin walled cellular vesicle which
communicates with the exterior by a pore on the dorsal surface.
This vesicle is of similar character and origin to the peritoneal
vesicle of the Echinoderm larva (see INVERTEBRATE EMBRYOLOGY)
and it becomes the cavity of the proboscis of the adult, still
municating with the exterior by a pore. Behind it lies a pulsating
vesicle or "heart" and a similar vesicle is found in the Echinoderm.
A pair of peritoneal vesicles is developed, as the larva grows
older, in the collar region between the gut and the skin, and another such pair in the trunk. The pulsating vesicle persists as the pericardium and heart in the base of the proboscis. There seems to be no escape from the conclusion that Balanoglossus and the Echinodermata are widely divergent offshoots from the same stock. But is Balanoglossus really related to the Vertebrata and does it therefore form a link between these two great groups? Now in all Vertebrata besides the gill-slits, which even in land animals appear during embryonic life, there are two other dis tinguishing features, viz., (I) the notochord, a gelatinous rod developed from the mid-dorsal cells of the gut-wall round which the "vertebrae" are later built up ; and (2) above the notochord a hollow tubular nerve cord. Bateson has shown that both these structures are represented in Balanoglossus (fig. 3), although re stricted to the collar and neck, joining the collar with the proboscis. The notochord is a hollow outgrowth from the dorsal wall of the gut, surrounded by a tough cuticular membrane, espe cially underneath; it projects into and supports the neck of the proboscis. The nervous system over the whole surface of the proboscis and along the mid-dorsal and mid-ventral lines of the trunk is a mere plexus or nerve net lying at the base of the skin, but in the collar it is a deeply embedded tube opening to the exterior in front and behind. Subsequently it was shown by the body. Whilst in the condition represented by the tornaria larva, the ancestral stock glided through the waters by means of cilia; in Balanoglossus which is perhaps a degenerate offshoot, the powers of locomotion are concentrated in the proboscis and collar.
(E. W. MACB.)
