Sagitta: An Example of Phylum Chaetognatha

In this article we will discuss about Sagitta:- 1. Habit and Habitat of Sagitta 2. External Structures of Sagitta 3. Body Wall 4. Coelom 5. Digestive System 6. Circulatory, Respiratory and Excretory Systems 7. Nervous System 8. Reproductive System 9. Development.

Habit and Habitat of Sagitta:

Sagitta is a marine planktonic animal which lives throughout the world. It can swim very slowly and most of the time it floats on the surface of the sea.

The fins help in floatation. Sagitta is a predaceous form and captures small planktonic animals as food by head armature. It takes food mostly at night. Sagitta exhibits diurnal migration. It comes to the surface at night and descends down during daytime. This migration results possibly as a reaction to light or temperature.

External Structures of Sagitta:

Sagitta has an elongated torpedo-like, bilaterally symmetrical body (Fig. 22.1A). The body is divisible into three parts—head, trunk and tail. The head is more or less triangular in outline and is separated from the trunk externally by a constricted region called neck and internally by a coelomic septum.

The head bears four rows of teeth, one pair of anterior teeth and another pair of posterior teeth. These teeth help in food capture. Posterior to the teeth on the dorsal side of the head, there exists a pair of ves­tibular organs and a glandular depression, called vestibular pit, behind the vestibular organs.

A pair of pigmented eye spots is present on the dorsal surface of the head. There is a depression at the centre of the ventral side of the head, called vestibule, which leads into a longitudinal slit, called mouth. On the lateral side of the mouth there are 4-14 grasping spines (seizing jaws or prehensile spines). These are operated by powerful muscles and help in the capture of prey.

The head is covered over by a hood on the dorsal side. This hood is fin-like exten­sion of the body with coelomic cavities. The hood is attached ventrally near the neck region. The hood forms a protective device over the food-catching apparatus. The trunk is slightly broader towards the middle re­gion and is provided with two pairs of lateral fins.

The fins are thin and transparent extension of epidermis of the body wall. The fins are devoid of muscles and act as the floating organ. The trunk is internally demar­cated from the tail by a coelomic septum behind the anus. The tail is provided with a caudal fin.

Body Wall of Sagitta:

The body wall is composed of an outer epidermis which secretes a thin cuticle (Fig. 22.1B). The epidermis consists of epidermal cells, the shapes of which may vary at differ­ent regions. In Sagitta, a peculiar structure, called corona ciliata or ciliary loop is present at the anterior end as a dorsal strip of modi­fied epidermis. It forms a ridge over the epidermis proper where two or three rows of cells bear long fine cilia.

Beneath the epider­mis, a thin basement membrane exists upon which the epidermis rests. The body wall musculature consists of straited muscle layer which forms two dorso-lateral muscular bands. In the head region, transverse as well as oblique muscle are also present. The histo­logical organisation of the muscle resembles closely with the muscles of the nematodes.

Coelom of Sagitta:

The nature of the coelom is not properly known in Sagitta, although a distinct space is present between the body wall and the ali­mentary tube. In the embryonic stage, coe­lom develops by enterocoelous method but its subsequent development is stopped at later stages. As a consequence, the homology of the adult coelom remains disputed.

The head coelom is separated from the trunk coelom by a coelomic septum and the trunk coelom is also demarcated from the tail coelom by a similar coelomic partition. Both the trunk and tail coeloms are subdivided by dorsal and ventral mesenteries. In this genus the tail coelom is further subdivided by lateral mesenteries. In Sagitta, the coelom lacks the peritoneal lining.

The coelomic fluid is colourless. It lacks definite cells, but minute floating granules are present.

Digestive System of Sagitta:

The digestive tract of Sagitta is a straight tube running between the mouth and anus. The mouth is situated at the ventral side of the head and is provided with grasping teeth acting as jaws. The mouth leads into a muscular pharynx which be­comes dilated posteriorly as pharyngeal bulb.

The pharynx is lined by granular se­cretory cells, but in the pharyngeal bulb the cells have compound granules. True stomach is absent. The pharynx passes down to the straight intestine. The intestine, just near its commencement, gives off a pair of diver­ticula on the lateral sides. The intestine ter­minates into the anus situated at the junction of trunk and tail.

Circulatory, Respiratory and Excretory Systems of Sagitta:

In Sagitta, there is no trace of three systems, such as, circulatory, respiratory and excretory systems. With all possibilities, the coelomic fluid plays Ike physiological role of circulation, respiration and excretion.

Nervous System of Sagitta:

The nervous system is fairly developed in Sagitta. It consists of a large cerebral gan­glion or brain on the dorsal side of the pharynx and an elongated sub-enteric or ventral ganglion beneath the pharynx.

Both the ganglia are connected by circumenteric connectives. Two pairs of ganglia—one pair of vestibular ganglia and another pair of pharyngeal ganglia are connected with the brain. From these ganglia nerves spread out to the various parts of the body.

Sense Organs:

The organs of special senses are present in Sagitta.

The sense organs are:

(1) Sensory bristles:

Longitudinal rows of elongated cells with delicate bristles are regarded as tactile organs.

(2) Ciliated loop:

The ciliated loop with all probabilities, acts as chemoreceptor.

(3) Eyes:

Two eyes are present on the dorsal side of the head. Each eye has five closely applied pigment-cup ocelli.

Reproductive System of Sagitta:

Like all other Chaetognaths, Sagitta is bisexual. The ovaries and testes are paired structures. The ovaries are elongated solid bodies situated in the posterior part of the trunk coelom just in front of the trunk-tail coelomic septum and remain attached with the body will by mesentery. The oviduct is very peculiar in having two tubes—one tube within another.

The inner tubes has a syncy­tial wall and it becomes expanded posteriorly to form seminal receptacle. The wall of the oviduct proper, i.e., the outer tube is com­posed of columnar or cuboidal epithelium. The transportation of the egg from the ovary to the oviduct is very peculiar in Sagitta.

At the time of maturity of each egg, two cells from the inner tube of the oviduct move and make an attachment stalk tor Ike egg. This attachment stalk becomes subsequently hol­low to form a narrow canal through which spermatozoa from the seminal receptacle migrate and fertilize the egg.

The testes are located in the tail coelom just posterior to the trunk-tail coelomic par­tition. At the time of sexual maturity, clusters of spermatogonia become detached from the testes into the tail coelom where sperma­togenesis occurs.

The vas deferens has a funnel-like coelomic opening, called coelomostome, and the other end dilates to form vesicula seminalis. There is no perma­nent male gonopore, the filiform sperms escape by rupturing the body wall.

Development of Sagitta:

Fertilization is internal. In Sagitta, self- fertilization is the rule, but in other Chaetognaths cross-fertilization occurs. The egg contains little quantity of yolk material, the egg undergoes holoblastic or equal clea­vage and a coeloblastula is formed.

The coeloblastula eventually gastrulates in em­bolic fashion. The most significant event in the development of Sagitta is the precocious differentiation of primordial germ cells. The embryo becomes elongated and hatches out of the egg shell as a miniature animal. Thus, in Sagitta the development is direct.