t the
truncal muscles are of subordinate importance in locomotion, serving
principally to promote the peristaltic contractions of the body by which
the food is carried through the gut. The function of alimentation is
closely associated with that of locomotion, somewhat as in the burrowing
earthworm; in the excavation of its burrows the sand is passed through the
body, and any nutrient matter that may adhere to it is extracted during its
passage through the intestine, the exhausted sand being finally ejected
through the vent at the orifice of the burrow and appearing at low tide as
a worm casting. In accordance with this manner of feeding, the mouth is
kept permanently open and prevented from collapsing by a pair of skeletal
cornua belonging to a sustentacular apparatus (the nuchal skeleton), the
body of which lies within the narrow neck of the proboscis; the latter is
inserted into the collar and surrounded by the anterior free flap of this
segment of the body.

When first discovered by J. F. Eschscholtz at the Marshall Islands in 1825,
_Balanoglossus_ was described as a worm-like animal belonging to the
Echinoderm order of Holothurians or sea-cucumbers. In 1865 Kowalevsky
discovered that the organs of respiration consist of numerous pairs of
gill-slits leading from the digestive canal through the thickness of the
body-wall to the exterior. On this account the animal was subsequently
placed by Gegenbaur in a special class of Vermes, the Enteropneusta. In
1883-1886 Bateson showed by his embryological researches that the
Enteropneusta exhibit chordate (vertebrate) affinities in respect of the
coelomic, skeletal and nervous systems as well as in regard to the
respiratory system, and, further, that the gill-slits are formed upon a
plan similar to that of the gill-slits of _Amphioxus_, being subdivided by
tongue-bars which depend from the dorsal borders of the slits.

_Coelom and Pore-canals_.--In correspondence with the tri-regional
differentiation of the body in its external configuration, the coelom
(body-cavity, perivisceral cavity) is divided into three portions
completely separated from one another by septa:--(1) proboscis-coelom, or
first body-cavity; (2) the collar-coelom, or second body-cavity; (3)
truncal coelom, or third body-cavity. Of these divisions of the coelom the
first two communicate with the exterior by means of a pair of ciliated
pore-canals placed at the posterior end of their respective segments. The