SKELETON OF HEMICHORDATA, UROCHORDATA, AND CEPHALOCHORDATA.
SUBPHYLUM A. HEMICHORDATA.
THE subphylum includes three genera, _Balanoglossus_[19],
_Cephalodiscus_ and _Rhabdopleura_; and perhaps a fourth, _Phoronis_.
The skeletal structures found in _Balanoglossus_[20] are all
endoskeletal. They include:
(1) The =notochord=. This arises as a diverticulum from the alimentary
canal which grows forwards into the proboscis and extends beyond the
front end of the central nervous system. It is hypoblastic in origin
and arises in the same way as does the notochord of _Amphioxus_. Its
cells become highly vacuolated and take on the typical notochordal
structure[21]. The cavity of the primitive diverticulum becomes
obliterated in front, but behind it opens throughout life into the
alimentary canal.
(2) The =axial skeletal rods=. These are a pair of chitinous rods
which lie ventral to the notochord and in the collar region unite to
form a single mass.
(3) The =branchial skeleton=. The gill bars separating the gill slits
from one another are strengthened by chitinous rods in a way closely
similar to that in _Amphioxus_. But between one primary forked rod and
the next there are two secondary unforked rods--not one, as in
_Amphioxus_.
(4) The =chondroid tissue=. This is of mesoblastic origin and may be
regarded as an imperfect sheath for the notochord.
In _Cephalodiscus_ and _Rhabdopleura_ as in _Balanoglossus_ the
notochord forms a small diverticulum growing forwards from the
alimentary canal into the proboscis stalk.
Recent researches on _Phoronis_[22] show the existence in the collar
region of the larva (_Actinotrocha_) of a paired organ, which is
regarded by its discoverer as representing a double notochord.
SUBPHYLUM B. UROCHORDATA (TUNICATA).
Skeletal structures of epiblastic and hypoblastic origin occur in the
Urochordata. Most Tunicates are invested by a thick gelatinous test
which often contains calcareous spicules, and serves as a supporting
organ for the soft body. The cells of this test are mesodermal in
origin.
In larval Tunicata and in adults of the group Larvacea the tail is
supported by a typical notochord, which is confined to the tail. In
all Tunicata except Larvacea all trace of the notochord is lost in the
adult.
SUBPHYLUM C. CEPHALOCHORDATA.
[Illustration FIG. 3. DIAGRAM OF THE SKELETON OF _Amphioxus
lanceolatus_ × 3 (after a drawing in the Index collection at the Brit.
Mus.).
1. skeleton of dorsal fin.
2. notochord.
3. neural tube.
4. buccal skeleton.
5. branchial skeleton.
6. septa separating the myotomes.
7. skeleton of ventral fin.]
This subphylum includes the well-known genus _Amphioxus_[23]. In
_Amphioxus_ the skeleton is very simple. It contains no trace of
cartilage or bone and remains throughout life in a condition
corresponding to a very early stage in Vertebrata. The skeleton of
_Amphioxus_ is partly hypoblastic, partly mesoblastic in origin.
(_a_) =Hypoblastic skeleton.=
The =notochord= (fig. 3, 2) is an elastic rod extending along the
whole length of the body past the anterior end of the nerve cord. It
lies ventral to the nerve cord, and shows no trace of segmentation. It
is chiefly made up of greatly vacuolated cells containing lymph, but
near the dorsal and ventral surfaces the cells are less vacuolated.
The notochord is immediately surrounded by a structureless cuticular
layer, the _chordal sheath_, and outside this comes the mesoblastic
_skeletogenous layer_, which also surrounds the nerve cord.
The =branchial skeleton=. This consists of a series of chitinous
elastic rods which strengthen the gill bars and are alternately forked
and unforked ventrally. The forked rods are primary, and are U-shaped
in section, the unforked rods are secondary, and are circular in
section. All these rods are united at intervals by transverse rods.
(_b_) =Mesoblastic skeleton.=
The =buccal skeleton=. On each side of the mouth there is a curved bar
resembling the notochord in structure. The bars are segmented, and
each segment bears a smaller rod which supports a tentacle, the whole
forming the buccal skeleton (fig. 3, 4).
The notochord is enclosed in a thick =sheath= of connective tissue
continuous with a thinner sheath round the nerve cord. The sheaths of
the notochord and nerve cord together form the skeletogenous layer,
and prolongations of it form the myomeres or septa between the
myotomes or segments of the great lateral muscles of the body.
The =skeleton of each median fin= consists of small cubical masses of
a gelatinous substance arranged in rows (fig. 3, 1 and 7), and serving
to strengthen the fins.
FOOTNOTES:
[19] The name _Balanoglossus_ is used here in its widest sense to
include all the Enteropneusta.
[20] See W. Bateson, _Quart. J. Micr. Sci._ n. s. vol. XXIV. 1884, p.
208 and later; also E.W. Macbride, _Ibid._ vol. XXXVI. 1894, p. 385.
[21] See p. 52.
[22] A.T. Masterman, _P.R. Soc. Edinb._ 1895-96, p. 59; and _Anat.
Anz._ 1896, p. 266.
[23] See E. Ray Lankester, _Quart. J. Micr. Sci._ vol. XXIX. n. s.
1889, p. 365. W.B. Benham, _Ibid._ vol. XXXV. n. s. 1893, p. 97. J.W.
Kirkaldy, _Ibid_. vol. XXXVII. n. s. 1895, p. 303. The last-named
writer divides the genus into three subgenera.