Wesley R. Elsberry I just got a little scanner and OCR software package, which means that
Wesley R. Elsberry
I just got a little scanner and OCR software package, which means that I
can now transcribe some of my notes on invertebrate zoology. Here's the
synopsis of Phylum Chaetognatha.
I. HISTORICAL ACCOUNT:
A. Slabber (1769) first reported on these animals calling them Zee
worm (sea worm) or Sagitta or pyl (arrow) nd assigned them to
Linnaeus's group Intestina, an order of Vermes.
B. Scoresby (1820) included two figures of Sagitta in a plate
illustrative of the food of whales but made no mention in the text.
C. Darwin (1844) published saying the species Sagitta was
"remarkable for obscurity of their affinities" and was first to des-
cribe correctly the grasping spines and their action.
D. In the same year Krohn discussed the difficulty of placing
Sagitta systematically and leaned toward an annelid affinity opposing
the molluscan affinity proposed by some of his predecessors.
E. T. H. Huxley (1851) rejected the molluscan affinity and proposed
an arthropodan relationship.
F. Gegenbaur (1853) reported on the embryology of Sagitta and
declared the absence of resemblance to molluscan development however
he offered no suggestions.
G. Leuckart (1854) proposed to regard the genus as constituting
a separate group. Chaetognathi, placed between nematodes and oligo-
H. A. Schneider (1866) allied the chaetognaths with the nema-
todes in phylum Nemathelminthes.
I. Metschnikov purported to find great similarity between
Sagitta and the draconematid and epsilonematid marine nematodes
claiming direct relationship between spines and head bristles and
between fins and stilt bristles.
J. From 1870 onward many studies were undertaken focusing on
the embryology of the animals, especially the mode of coelom forma-
tion whether enterocoelous or schizocoelous.
K. Grasse (1883) recounted the anatomy, histology, and
embryology and made careful comparisons with other metazoan groups
finally concluding that it was impossible to ally thechaetognaths with
any of them.
L. MacBride (1914) promoted the theory that the chaetognaths are
relatively unmodified offshoots of a primitive protocoelomate stock
from which all coelomate animals derive.
M. In the leading German textbook of zoology, the Claus-
Grobben-Kuehn Lehrbuch der Zoologie the chaetognaths are made an
independent branch under the phylum Deuterostomia. Hyman concurs but
states that systematic positioning and relationships remain
II. DEFINITION OF THE PHYLUM: The Chaetognatha are small, bilaterally
symmetrical enterocoelous marine animals, of mostly planktonic habits
without circulatory or excretory systems and with a slender, trans-
parent torpedo-shaped body, provided with one or two pairs of lateral
horizontal fins, and terminating anteriorly in a rounded head armed on
each side with a group of grasping spines, posteriorly in a horiz-
ontal tail fin.
III. EXTERNAL ANATOMY:
A. General Characteristics: Slender elongated, torpedo-shaped
creatures ranging from a few to 100 mm in length with the majority of
individuals measuring less than 40 mm. Display perfect bilateral
symmetry. Body is usually stiff, turgid and capable of only slight
bending. Body regionated into head, trunk, and tail.
B. Head Region.
1. Rounded or triangular, well demarcated externally by
onstricted neck area and internally by coelomic partition.
2. Armed anteriorly by two to four short rows of small teeth or
spines called anterior or posterior teeth (Fig. 1B). Number of teeth
varies with species and age.
3. On ventral surface of head just behind posterior teeth is
located a pair of vestibular organs (transverse ridge bearing papillae
(Fig. 1B). In many chaetognaths a vestibular pit (glandular
depression) is located behind vestibular organs. Centrally located
vestibule leads into mouth.
4. On dorsal surface of head are located retro cerebral organ and
a pair of pigmented eyes (Fig. 3F). In many, the ciliary loop extends
to this pore.
5. Grasping spines, prehensile spines, seizing jaws, etc.,
located on posterior part of head. Numbers range from 4 to 14. (Fig.
6. Hood - unique feature, a fold of body wall containing coelomic
space that can be drawn over the head (Fig. 1B). Withdrawn it leaves
the spines aad teeth free; drawn over the head, it protects the
food-catching apparatus and reduces the resistance of the head in
C. Trunk Region.
1. Slender, slightly fusiform broadening toward the middle or
toward the tail region. Delimited from head by head-trunk septum and
slightly narrowed neck. On both sides of neck region is thickened
stratified epidermis (Fig. 3C), which forms the collarette, which may
extend for some distance along the trunk.
D, Tail Region.
1. Demarcated from trunk by a post anal coelomic partition
2. Provided with two pairs of lateral fins in Sagitta zahonya and
some species of Spadella and one pair in others. The fins are thin,
transparent horizontal epidermal extensions supported by a double set
of delicate fin rays, dorsal and vent somewhat triangular in shape.
Tail fin is this horizontal ex- pansion with no musculature and
therefore incapable of swimming movements. Functions as floating and
IV. Body Wall:
A. Extremely thin cuticle secreted by a mostly one-layered
epithelium (Fig. 3B).
B. In some body regions epidermis is stratified and conists of
large bladdery cells (Fig. 3C).
C. In some areas the epidermis is glandular, lacking a cuticle.
Hood lining, vestibular pits.
D. Possibly glandular is the retrocerebral organ - a pair of
sacs imbedded in the posteriorpart of the cerebral ganglion and
separated from the nervous system by a membrane.
E. Benthonic species have adhesive papillae which are epidermal
projections located on ventral surface of the tail that act as sucker.
F. Hardened structures of the head: teeth, grasping spines and
lateral and ventral plates are epidermal projections. Only the
grasping spines are composed of chitin.
G. The ciliary loop is a dorsal strip of altered epidermis
usually lying immediately behind the retrocerebral pore and extending
posteriorly to the neck region or well back onto the trunk. The loop
is considered by most to be a sense organ.
H. Basement or basal membrane - very thin over most of the body,
thickened to form capsules for the eyes at the neck, and sometimes at
the trunk to form skeletal plates. Also, thickened in the fins,
the fin rays appearing to be extensions of the membrane.
A. Located beneath basement membrane; consists of a longitudinal
layer only and lacks a peritoneal membrane on the inner surface (con-
trary to what is normally expected in coelomate").
B. Body-wall musculature consists of four longitudinal: two
dorsolateral and two ventrolateral.
C. Head musculature much more complex controlling operation of
teeth, grasping spines, hood, vestibule and mouth.
D. All body-wall muscles are cross-striatad and are
histologically similar to vertebrate muscles:
A. A true coelom is formed during the embryonic stages but s
suppressed during larval development casting doubt that the cavity
which later reappears corresponds to the embryonic cavity.
B. No peritoneal lining.
C. Coelomic space of trunk and tail is subdivided into paired
ateral compartments by a mesentery that in the trunk enclosed the
ntestine between its two walls (Fig. SC).
D. Coelomic space of the tail is considered to be of secondary
E. Coelomic space of the head is considered to be a primary
F. Chaetognaths, therefore, have definitive coelom consisting of
the head cavity and the paired trunk-tail cavities.
G. Coelomic fluid is colorless and contains spheres and granules.
It circulates by moving forward along the inner surface of the body
wall and backward along the median mesentery (Fig. 7A).
VII. NERVOUS SYSTEM :
A. Circumenteric ring similar to that of nematodes.
B. A large cerebral ganglion and several lateral ganglia
innervate the head and body of the animal.
C. From the sides of the cerebral ganglion spring two circum-
enteric connectives that proceed obliquely backward to the ventral, or
subenteric ganglion and more posteriorly there arise from the cerebral
ganglion a pair of optic nerves. Medial to these arise a pair of
coronal nerves to the ciliary loop (Fig. 7C).
D. In the trunk the midventral subenteric ganglion gives rise
to a large number of paired nerves which serve the muscles and sense
organs of the trunk and tail.
VIII. SENSE ORGANS:
A. Bristles or tufts regarded as tactile in nature (Fig. 2).
As many as 250 arranged in six or seven longitudinal rows in Sagitta
B. Ciliary loop - possib1y chemoreceptive. Evidences also
suggests tactile function or at least ability to detect water dis-
C. Eyes - Five combined pigment cup ocelli.
IX. DIGESTIVE SYSTEM:
A. Mouth - T-like or oval shaped. Thrust forward during feeding.
B. Pharynx - muscular tube, expands posteriorly to form bulb,
lining epithelium consists of secretory granular cells.
C. Intestine - straight tube proceeding to anus lined by
cuboidal to columnar epithelium composed of glandular and absorptive
D. Rectum - thick basement membrane followed by a layer of cir-
E. Anus - no special muscle for governing anal aperture.
X. REPRODUCTIVE SYSTEM:
A. Protandric hermaphrodites, ovaries mature after tail coeloms
are filled with sperm.
B. Ovaries situated in posterior part of the trunk just anterior
to the trunk-tail septum and a pair of testes in the tail just behind
tne septum (Fig. 2). Oviducts along the lateral side of each ovary
end posteriorly in separate seminal receptacles. These open to the
exterior through a short vagina and gonopore. Two gonopores are on
each side of the body in front of the trunk-tail septum. Fertilized
eggs seemingly don't exit through the seminal receptacle and female
gonopore rather, some sort of temporary duct arises.
C. Testes are band line bodies in the anterior part of the tail
coelom. From each testis a sperm duct terminates in a seminal vesicle
imbedded in the lateral body wall. Spermatogonia bud off the testes
and spermatogenesis is completed in the coelom. Mature sperm pass
into the sperm duct and then to the seminal vesicle.in which the sperm
mass becomes coated with a secretion thus forming a spermatophore
which escapes by rupture.
XI. BREEDING HABIT5:
A. Self-fertilization - attachment disk is secreted for the
spermatophore. After rupture the disk adheres to the fins and
clusters of sperm eventually reach the seminal receptacles.
B. Cross fertilization - one animal places spermatophore near
other's female gonopore and roles are reversed.
C. Eggs may float beneath the surface, be attached to objects or
be attached to the animals back.
D. Breeding takes place throughout the year.
A. Holoblastic cleavage results in a coeloblastula. Radial,
B. Gastrulation by invagination, the anterior end wall of the
archenteron invaginates in the process creating two pair of lateral
D. Development from fertilization to hatching requires
about two days. Larval development is direct.
XIII. ECOLOGY AND PHYSIOLOGY:
A. All are planktonic except the benthonic Spadella.
B. Found in all seas and at all latitudes and through a range of
C. Mostly float motionless. Resort to swimming to maintain their
D. Swimming consists of short swift darts covering about 5 cm
that is followed by gliding and a return to motionlessness.
E. Highly predaceous, carnivorous, eating any animal of suit-
able size including fish and each other.
F. Digestion appears to be wholly extracellular.
2. Varying portions of the tail can be regenerated but
not exact duplicate.
A. About 65 described species most of which belong to the genus
Sagitta. Spadella only benthonic members.
B. No category higher than genus has been proposed.
A. Adult anatomy aligns them with Aschelminthes. However,
B. Embryology demonstrates they are true coelomates.
C. Hyman places them among the Deuterostomia because of .the
possibility that chaetognaths are related to the dipleurula ancestors
of the other Deuterostomia.
ALVARIN, Angeles 1965. Chaetognaths. Oceanogr. Mar. Biol.
Ann. Rev. (3) 115-194.
BARNES, Robert D. 1979. Invertebrate Zoology. Fourth Edition,
W. B. Saunders, Co., Phila. pps. 1046-1050.
COSPER, T. C. and M. R. Reeve 1970. Structural Details of the
mouthparts of a Chaetognath, as revealed by Scanning
Electron Microscopy. Vol. 20(2) 441-445.
HYMAN, L. H. 1959. The Invertebrates: Smaller Coelomate
Groups, Vol. V. McGraw-Hill Book Company, New York pp 1-71.
PIERCE, E. Lowe 1951. The Chaetognatha of the West Coast of
Florida. Biological Bulletin, 100(3): 206-228.
PIERCE, E. Lowe and Marvin L. Wass 1962. Chaetognatha from
the Florida Current and Coastal Water of the Southeastern
Atlantic States. Vol. 12, No. 3, 403-431.
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