Study Notes on Scolopendra

In this article we will discuss about Scolopendra:- 1. Habit and Habitat of Scolopendra 2. External Structures of Scolopendra 3. Integumentary System 4. Locomotion 5. Digestive System 6. Respiratory System 7. Circulatory System 8. Excretory System 9. Nervous System 10. Reproductive System.

Contents:

1. Habit and Habitat of Scolopendra
2. Exernal Structures of Scolopendra
3. Integumentary System of Scolopendra
4. Locomotion of Scolopendra
5. Digestive System of Scolopendra
6. Respiratory System of Scolopendra
7. Circulatory System of Scolopendra
8. Excretory System of Scolopendra
9. Nervous System of Scolopendra
10. Reproductive System of Scolopendra

1. Habit and Habitat of Scolopendra:

The common Indian Scolopendra is sci­entifically known as Scolopendra amazonica. The other Indian species are Scolopendra morsitans, Scolopendra valida, Scolopendra hardwickei, Scolopendra mazbii and Scolopendra subspinipes.

These elongated animals are photonegative and live in darkness. Spe­cially the burrows, crevices, logs of wood and fallen leaves are its favourite abode. It is a swiftly moving animal and prefers to stay in damp places. Scolopendra cannot stand temperature over 40 °C.

2. External Structures of Scolopendra:

The elongated and dorsoventrally flat­tened body is clearly divisible into a small head and a long trunk (Fig. 18.35). The body is covered by cuticular coating which is frequently shed off.

These exoskeletal cover­ings are comprised of numerous plates hav­ing varied appearances. These are known as sclerites. The arthroidal membranes con­nect these sclerites and allow limited free movement. The dorsal part is deep red and the ventral portion is yellowish in colour.

1. Head:

The head may be separated into cephalic capsule, cephalic lobes and ce­phalic appendages (Fig. 18.35). The cephalic capsule is enclosed dorsally by a cephalic plate and by several sclerites in ventral, lateral and anterior sides. It looks more or less circular from the dorsal side and trian­gular ventrally.

Anteriorly, the capsule of the head bears a pair of antennae which are many-jointed and set apart. Immediately behind antennae, on each antero-dorsal side two pairs of simple eyes are present. The cephalic or head lobe is divisible into labrum and hypopharynx.

These two, together with following appendages constitute the mouth parts (Fig. 18.36):

(i) A pair of horizontally placed mandibles with five teeth and bristles at their free end and

(ii) Two pairs of maxillae. The small first maxillae are fused with labium-like structure and consisting of two segmented palp and massive coxae. The basal piecces or coxae of the second maxilla unite with its fellow from the corresponding side to constitute lower lip. The remaining part of the second maxilla forms a three-jointed leg-­like palp with claw at the terminal end.

In addition to these appendages maxillipeds of first trunk segment work as mouth part. It is to be noted that the head of embryonic scolopendra is metamerically seg­mented, which fuses in the adult.

2. Trunk:

The trunk is divided into twenty- four identical segments. The first abdominal segment is closely apposed with head and bears a pair of appendages, called maxillipeds or poison claws or toxicognaths. Each maxilliped is made up of five pieces of which the terminal one bears a sharp claw through which opens the poison gland.

The next twenty-one segments (2nd-22nd) bear in each a pair of legs. Each leg is seven-jointed (though five are distinctly visible) and termi­nated with a claw having two ventral spines. The legs gradually increase in size from anterior to posterior. The pair of legs in the 22nd segment which is called anal leg is longer than the remaining legs.

In males, this difference is well marked. The body is termi­nated by a piece which is not regarded as true segment. It is known as telson and it bears an aperture, the amis. In between 22nd segment and telson, the 23rd and 24th seg­ments have taken part in the making of external genitalia. In males, the genital seg­ment is provided with a pair of projections known as genital styles.

3. Integumentary System of Scolopendra:

The integument is composed of:

(1) Two- layered cuticle,

(2) Monolayered hypodermis and

(3) Non-cellular basement mem­brane.

The absence of waxy coating over the cuticle causes rapid loss of water in Scolopendra. For this reason, these animals prefer to stay in damp places and drink water regularly. Both unicellular and multi­cellular glands are seen to be associated with the integument.

The unicellular integumen­tary gland cells are scattered in the hypodermis and they produce a lipoid material to cover the external part. The poison gland in the maxilliped is a kind of unicellular gland. Here the cylindrical sac-like body of the gland is lined by numerous unicellular glands (Fig. 18.37).

These glands produce a yellow­ish, acidic fluid, which ultimately passes through a highly cuticularised duct to open in the centre of maxilliped claw. The secre­tion of poison gland is believed to have three functions—offensive, digestive and partially lubricating.

Following multicellular glands are seen in scolopendra:

A. Glands opening in the head region:

The glands mentioned below are prima­rily concerned with digestion. These are called salivary glands. Their products are also be­lieved to be used as cleansing fluid, and act as fungicide.

(1) Epipharyngeal glands:

These glands are two pairs and are placed beneath the brain. Anterior pair is larger and communicates laterally with each other. The outlets of their ducts are placed in epipharyngeal surface.

(2) Labial glands:

These are seen as a pair of prominent lobular glands in the space between fifth and seventh segments. Their products are collected by numerous small ducts, which in turn unite to form a common duct and extend anteriorly along the alimen­tary tube. It finally opens to the exterior through the ventral surface of the head and between the hypopharynx and labium.

(3) Maxillary glands:

One pair of these glands is present in the third and fourth segments. These glands excepting their sizes closely resemble the labial glands. Their ducts open near the second maxillae at the ventral side of head region.

B. Glands opening in the trunk region:

Exact functions of these glands mentioned below are not known:

(1) Coxal gland:

One pair of this gland begins from second pedal segment and extends up to fourth pedal segment. It is highly vesicular and each vesicle has its own duct. All the ducts unite to form a common duct which opens through the ventral side of the first leg bearing segment.

(2) Coxopleural glands:

Last pedal seg­ment bears numerous flask-shaped coxopleural glands. Each gland has a sepa­rate duct which opens to the exterior on the coxopleural plates.

4. Locomotion of Scolopendra:

Legs in scolopendra are meant for carry­ing the weight of the body and at the same time to provide the necessary force to bring change of position. Powerful muscles and stiff integument render the operation of leg feasible.

During locomotion when legs of one side are lifted from the ground the opposite row touches it. In the same row, a leg rests on the ground immediately when the preceding leg is lifted. This results into a harmonious operation of legs and allows the animal to move forward in zigzag fashion.

5. Digestive System of Scolopendra:

The digestive or alimentary system in­cludes alimentary canal and digestive glands (Fig. 18.38). The alimentary canal is divisible into three parts—fore gut, mid gut and hind gut. In front of the fore gut there is another chamber, called pre-oral cavity, which hides the mouth.

The fore and hind guts have cuticular lining but mid gut is devoid of it. The fore gut is the longest part and extends up to 13th leg-bearing segment. It opens through mouth which remains at the ante­rior end and is practically concealed by the appendages.

The mouth part is formed in such a way that it is forwardly directed and the parts cover each other from behind for­wards. For this reason the head of Scolopendra is called prognathus type. The remaining part of the fore gut consists of pharynx, oesophagus, crop and proventricu­lus.

The pharynx is a broad chamber with strong muscular wall. In transverse section it appears ‘H’-shaped. It leads into the oesopha­gus which is a short tube and passes to an elongated thin-walled crop. The canal again becomes narrow and enters within a swollen proventriculus.

The proventriculus opens in the mid gut which runs from 13th-19th trunk segments. The first part of the hind gut which receives mid gut is broad and a pair of long Malpighian tubules opens near its be­ginning. The last part is narrow and finally communicates to the exterior through anus.

The glands opening in the head, i.e., epipharyngeal, labial and maxillary, are the principal digestive glands of Scolopendra. They are collectively called salivary glands. The lining of the mid gut contains special types of epithelial cells which also liberate digestive enzymes.

The Scolopendra is om­nivorous. It preys on small insects, spiders and even the members of its own group. The mouth parts help in food getting. It also drinks water regularly to prevent the water loss. When hungry, the scolopendra is fero­cious, but after a full meal it becomes slug­gish and docile.

6. Respiratory System of Scolopendra:

Nine pairs of triradiate slit-like spiracles are present on the posterior and dorsolateral sides of the 3rd, 5th, 8th, 12th, 14th, 16th, 18th and 20th pedal segments. Each spiracle leads into trachea, which has spiral fibres.

The tracheae branch extensively and anastomose within the different parts of the body. When at rest the spiracular muscles cause respira­tory movements, but during locomotion this job is also carried by other body muscles.

7. Circulatory System of Scolopendra:

This system includes a circulating fluid, called blood, which flows through blood ves­sels and blood sinuses. The blood of Scolopendra is a light-yellowish fluid, in which are suspended a few corpuscles called haemocytes. The corpuscles are either oval or spherical or spindle-shaped and are phago­cytic in nature.

The blood of Scolopendra performs the following services:

(1) Trans­port of food matters,

(2) Carrying of waste materials,

(3) Convey of pressure from one end to the other and

(4) Reservoir of water.

The blood vessels include:

(1) Dorsal vessel or heart and

(2) Ventral or supraneural vessel.

The heart is an elongated muscular tube having no epithelial lining. It extends from 2nd to 21st leg-bearing segments and in the 21st segment it has become abruptly slender. The major portion of the heart (3rd to 21st segments) remains within a thin cardiac dia­phragm.

The wall of the diaphragm is com­posed of two non-cellular membranes with pericardial tissue in between. In each seg­ment the diaphragm is drawn into two points, with which it remains connected with the tergum of the segment. The diaphragm is provided with a special kind of fan-shaped muscle, called alary muscles.

In each segment the heart communicates with the pericardial cavity by a pair of ostia and sends a pair of branches. The ventral or supraneural vessel also begins from second leg-bearing segment and extends up to the last segment. It runs immediately above the central nerve cord. The dorsal and ventral vessels at their ante­rior ends are connected by a ring-like circumstomodaeal vessel which anteriorly sends four cephalic vessels—one dorsal, one ven­tral and two laterals.

These vessels supply blood to the different parts of the anterior region—maxillipeds, muscles, antennae, brain and pharynx. There are numerous small sinuses which collect and drain blood within two large sinuses. One such sinus is called dorsal sinus and the other is known as perivisceral sinus.

The dorsal sinus is present on the dorsal side between cardiac diaphragm and body wall. The dorsal sinus communi­cates with the ventral part of the body through the gaps of the diaphragm. The perivisceral sinus is a space around the gut which is enclosed within a perforated membrane.

Mechanism of Circulation:

The heart contracts (systole) and relaxes (diastole) rhythmically and provides the force to drive the blood. At the time of diastole, heart receives blood from dorsal sinus through ostia.

The systole appears as a wave­like contraction from the posterior to the anterior end and drives the blood to the various parts of the body through different vessels. The blood after flowing through smaller sinuses is collected within the dorsal sinus and again returns to the heart.

8. Excretory System of Scolopendra:

The excretory organs are known as Mal­pighian tubules. Two such tubules originate from the beginning of hind gut and run anteriorly. Each tubule is spirally coiled and placed on either side of the alimentary canal. The tubule has a lining of columnar epithe­lial cells and its lumen contains a fluid and floating degenerated cells.

In addition to Malpighian tubules following structures are also known to work secondarily as excretory organs:

A. Lymphatic threads:

Present around Malpighian tubules and are blue in colour.

B. Kowalevsky’s corpuscles:

Blue col­oured small structure remains within fat body. It can absorb foreign particles like bacteria. It is believed that in addition to its excretory function it is also responsible for producing blood cells.

C. Fat bodies:

These structures are mainly concerned with storage of reserve materials. But they are also known to have some excre­tory functions.

9. Nervous System of Scolopendra:

The nervous system consists of central nerv­ous system, peripheral nerves and sense organs.

The central nervous system is represented by:

(a) Cerebral ganglia, a prominent two-lobed struc­ture in the head,

(b) Sub-oesophageal ganglion on the ventral side of the head region,

(c) Oesopha­geal commissures, one from each side of the cerebral ganglion and uniting with the sub-oesophageal ganglion and

(d) Ventral nerve cord which begins from sub-oesophageal gan­glion and runs posteriorly along the middle line.

In each segment, there is a ganglion on the nerve cord. The ganglia of 1st and 2nd seg­ments are closely set together and the genital ganglion is placed after the last segment.

The cerebral ganglion sends a visceral nerve to the alimentary canal. In addition, it innervates antennae, eyes and other associ­ated structures in the head. The sub-oesophageal ganglion sends nerves to the jaws and maxillipedes. The ventral ganglia send pe­ripheral nerves to the various structures in the corresponding segment.

The sense organs include:

(1) Sensory hairs and

(2) Spines, which are scattered all over the body.

In addition, there are:

(3) Maxillary organs inside the base of 1st maxilla and

(4) Organs of Tomosvary in the base of antenna. The latter is believed to be auditory in function.

But the most important sense organs are:

(5) eyes or sensila optica. These are paired and simple in construction.

10. Reproductive System of Scolopendra:

In both the sexes, the gonads are located on the mid-dorsal side of the posterior end (Fig. 18.39).

In males, the testes are twenty in number. Each testis is spindle-shaped and opens within a short vas efferens. All the vasa efferentia communicate with a centrally placed vas deferens.

The vas deferens divides into two, encircles the gut and again reunites. The last part of the duct is coiled and opens to the exterior through an aperture on the ventral side and just before anus. Two pairs of acces­sory glands are associated with the male reproductive system.

In the last pedal seg­ment the external genitalia in male are fused with post-genital and anal parts by a mem­brane. The external genitalia are represented by an opening, called gonopore, a round plate, called genital sternite with two genital appendages.

The post-genital part forms the ventral side of the copulatory organ. The copulatory organ is formed by the fusion of two sclerites and receives gonopore and the ducts of accessory glands.

In females, the ovary is a solitary tubular structure and extends along the mid-dorsal line over the alimentary canal. Like the vas deferens, the oviduct also divides and reunites.

The last part of the oviduct possesses a pair of receptacula seminalis and two pairs of accessory glands. Female genitalia consist of two parts—genital and anual. The oviducts open within genital atrium which is placed inside the body and covered by genital sternites.

During reproduction, the males release sperm cells in packets which are called spermatophores. But the mechanism of their transfer within female is not known. The ovum is covered by a chorion and contains scattered yolk granules.

The fertilization and part of the develop­ment are internal. At a time a female Scolopendra lays 15-33 eggs and burrows them in the earth 308 cm deep. Each egg is nearly 3 mm in length and the entire clutch remains enveloped in a fluid medium.