Wesley R. Elsberry I just got a little scanner and OCR software package, which means that

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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. 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- chaetes. 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 problematical. 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. 4A). 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 forward swinming. 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 equilibratory aids. 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. V. MUSCULATURE: 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: VI. COELOM: 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 formation. E. Coelomic space of the head is considered to be a primary coelomic division. 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 elegans. B. Ciliary loop - possib1y chemoreceptive. Evidences also suggests tactile function or at least ability to detect water dis- turbances. 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 cells. D. Rectum - thick basement membrane followed by a layer of cir- cular muscles. 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. XII. EMBRYOLOGY: A. Holoblastic cleavage results in a coeloblastula. Radial, indeterminate. B. Gastrulation by invagination, the anterior end wall of the archenteron invaginates in the process creating two pair of lateral coelomic sacs. 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 depth. C. Mostly float motionless. Resort to swimming to maintain their level. 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. G. Regeneration. 1. Head 2. Varying portions of the tail can be regenerated but not exact duplicate. XIV. CLASSIFICATION: 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. XV. PHYLOGENY: 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. REFERENCES 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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