In this article we will discuss about Hagfish:- 1. Introduction to Hagfish 2. Habit and Habitat of Hagfish 3. External Structures 4. Digestive System 5. Respiratory System 6. Circulatory System 7. Nervous System 8. Excretory System 9. Reproductive System 10. Development 11. Affinities and Systematic Position.
Contents:
- Introduction to Hagfish
- Habit and Habitat of Hagfish
- External Structures of Hagfish
- Digestive System of Hagfish
- Respiratory System of Hagfish
- Circulatory System of Hagfish
- Nervous System of Hagfish
- Excretory System of Hagfish
- Reproductive System of Hagfish
- Development of Hagfish
- Affinities and Systematic Position of Hagfish
1. Introduction to Hagfish:
Several genera of Cyclostomes like Myxine (Fig. 5.18A-B), Notomyxine, Neo- myxine and Eptatretus (Bdellostoma) (Fig. 5.17) are commonly called hagfishes. Although the lampreys and the hagfishes are included under the same class. The hagfishes show a wide range of variation in structural construction.
2. Habit and Habitat of Hagfish:
Hagfishes are exclusively marine and spend most of the time in burrows excavated in sand or mud and usually devour polychaete worms and dead fishes. They probably attack the dying or just dead fishes and suck out the whole content except the bones and skin. The sucking apparatus is highly developed in hagfishes. Hagfishes are nocturnal animals.
3. External Structures of Hagfish:
Hagfishes have slender eel-like body measuring from 45 cm (Myxine) to 1 m (Eptatretus) in length. An inconspicuous continuous median dorsal fin extends over the mid-dorsal region of the body. It runs around the tail and proceeds anteriorly as a ventral fin up to the cloacal opening.
The caudal fin is also narrow and is of diphycercal type (the cartilaginous epichordal or dorsal and hypochordal or ventral rays are similarly developed on the two sides of the notochord) like that of lampreys. The mouth is terminal in position and is bounded by delicate lips. The buccal cavity is more extensive than that of lampreys, but the buccal funnel is absent.
There are about three or four pairs of tentacles around the edge of the mouth (Fig. 5.18A). The tentacles are supported by skeletal rods. Just on the ctorsal side of the mouth there is a curved large median tooth. There are two rows of teeth on the tongue. Secondary unpaired olfactory organs open to the exterior by a paired external nasal apertures.
These apertures open terminally and close to the mouth. The nasohypophysial tube opens into the buccal cavity. The paired eyes are rudimentary and functionless. The numbers of external openings of gill-pouches vary from 1 to 15 pairs. In myxine, the efferent ducts from the six gill-pouches of one side unite to form a common duct which opens to the exterior by a single slit (Fig. 5.18B).
So in Myxine only one pair of gill-slits are visible. But in Eptatretus there exist thirteen to fifteen separate external gill-slits on each side of the body (Fig. 5.18E). There are rows of minute openings of the mucous glands specially on the ventral side of the body.
Body wall and musculature:
The integument, like that of lampreys, is soft and lacks exoskeleton. The epidermis contains special types of thread cells in hagfishes (Fig. 5.19). Each such cell contains a continuous fibrous proteinaceous thread, which after release absorbs water to form a thick coating of mucus over the body.
The muscles in the body are arranged in myomeres. But in hagfishes, the myotomes are undivided, i.e., the myotomes of one side alternate with those of the other side of body. The ‘tongue’ is associated with an intricate system of muscles.
Locomotion:
The hagfishes are sluggish animals and remain buried in the sandy bottom. The power of locomotion is comparatively poor because of feeble development of the body musculature. However, the mechanism of locomotion in hagfishes is similar to that described in lampreys.
Skeletal structures:
The skull is fundamentally similar to that of lampreys, but the roof is entirely membranous. The notochord is en-sheathed by fibrous tissue, but the vertebral elements are absent. The nasal duct is covered by a fenestrated cartilaginous capsule and a series of rings which are attached with the cartilages of the skull.
The skeletal support of the branchial region is ill-developed. The branchial basket is represented by a few anterior branchial arches. The arches are the first brancial and extra-branchial cartilages and the second extra-branchial cartilages. A pair of rudimentary cartilages support the common gill-slits of Myxine.
4. Digestive System of Hagfish:
The alimentary canal is a straight tube with a comparatively wide intenstine. The ‘tongue’ is well-developed and is operated by elaborate tongue musculature. Two rows of small teeth are present on the ‘tongue’. These teeth arise from the dental plate. The ‘tongue’ and sucking apparatus are well-developed. The stomach is a slightly dilated chamber.
The intestine lacks the spiral valve. The liver is composed of two separate portions, one is dorsal and the other is ventral. A gall bladder is present. Two ducts, one coming from each portion of the liver, open independently into the gall bladder. A pancreas-like structure is represented by the aggregations of secretory tubules. It is comparable with that of lampreys.
5. Respiratory System of Hagfish:
The gills are modified into the gill- pouches. The branchial region in hagfishes is restricted to the roundish respiratory enlargements (gill-pouches) which open into the pharynx. The number of the gill-pouches vary from six to fifteen pairs in hagfishes. Myxine has six pairs of gill-pouches while Eptatretus possesses thirteen to fifteen pairs of gill- pouches.
In Myxine the gill-pouches do not open separately to the exterior as seen in the case of lampreys. Each gill-pouch produces an elongated efferent exit tube. Thus six exit tubes of one side unite to form a common canal which opens to the exterior by a single gill-slit (see Fig. 5.18C).
So in Myxine only one pair of external gill-slit is present. But in Eptatretus, each gill-pouch opens separately, as a consequence thirteen to fifteen pairs of external gill-slits are present (see Fig. 5.18E).
In hagfishes, a pharyngocutaneous duct is present immediately behind the last gill- pouch and on the left side. This is a communicating duct between the pharyngeal cavity and the exterior. The pharyngocutaneous duct opens into the left external branchial pore. This duct helps in the expulsion of larger particles from the pharynx and thus prevents the entry of such particles into the delicate gill-pouches.
6. Circulatory System of Hagfish:
The heart is S-shaped and is a four- chamber structure consisting of a sinus venosus, auricle, ventricle and an inconspicuous conus arteriosus (Fig. 5.20). The conus arteriosus is ill-developed but the truncus is well- represented. The arterial arches in hagfishes are almost similar to that of lampreys except that each aortic arch supplies the hemibranchs of a single gill-pouch (Fig. 5.21).
The retention of left common cardinal (ductus Cuvieri) and the conversion of the right one into a portal heart supplying blood to the liver are the peculiar features in the circulatory system of hagfishes. The blood is iso-osmotic with the sea-water.
The blood forming lymphoid tissue is scatteredly observed in the sub-mucous layer of the alimentary canal. The pronephros is also recorded to be one of the sources of formation of blood cells.
7. Nervous System of Hagfish:
The nervous system of hagfishes is unlike that of the lampreys. The spinal cord is en-sheathed exclusively by fibrous tissue. The brain is devoid of any choroid plexus. The myelencephalon is large and the rest of the sectors of the brain are ill-developed in comparison to that of lampreys. The olfactory lobes are larger. The ventricles of the brain are greatly reduced.
The cranial nerves are almost similar to that of lampreys with certain individual peculiarities. The oculomotor, trochlear and abducens nerves are absent or extremely reduced as a consequence of degeneration of the corresponding eye muscles.
The ninth and tenth nerves, like those of lampreys, are postcranial, i.e., the glossopharyngeal and vagus nerves do not emerge from the brain. Besides these two posterior pairs of nerves, all other cranial nerves originate from the brain. The dorsal and ventral roots of the spinal nerves become united and are not separate as seen in case of lampreys.
Sense Organs in Hagfish:
The paired eyes are rudimentary and sunk below the skin. The eyes lack nerves and muscles. The pineal eye is absent. The ear has only one semicircular canal (see Fig. 5.22D) and this canal contains two ampullae one at each end. The nasohypophysial tube opens internally into the roof of the pharynx.
8. Excretory System of Hagfish:
The pronephros is retained in adult hagfishes, which is hardly marked off from the mesonephros. The pronephros lies dorsal to the pericardial cavity. In Eptatretus, the pronephric duct is present, but Myxine lacks such a duct. The pronephric tubules have no communication with the exterior. The functional kidney which is of mesonephric type is peculiar in having segmental disposition.
9. Reproductive System of Hagfish:
The hagfishes are hermaphroditic animals. The anterior portion produces eggs and the hinder part is testis-like. They are protandric forms, i.e., the male gametes are produced prior to the maturation of the female gametes. The eggs are larger in size and are cylindrical in shape. The eggs are covered by a protective horny shell. The eggs are of telolecithal type having hooked filaments for anchorage at either pole.
10. Development of Hagfish:
The developmental events are known only in Eptatretus. The cleavage is meroblastic. The cleavage takes place at the animal pole of the fertilized egg and the embryo differentiates at the posterior margin of the blastodisc.
The process of development resembles closely that of lampreys except in the initial stages of nerve tube formation. In hagfishes (as seen in Eptatretus), the development is direct, i.e., the larval life is absent. The stages of embryonic development of Eptatretus are shown in Fig. 5.21.
11. Affinities and Systematic Position of Hagfish:
The cyclostomes constitute a curious group of animals and represent the most primitive craniates that are living today. The anatomy of the cyclostomes exhibits many primitive, specialised and degenerative features which resemble other forms graded lowly or highly than this group. Because of the fact, the systematic position becomes a questionable issue.
Relationship with Limulus:
Patten and Gaskell tried to draw affinities between Limulus and Ammocoetes larva of lamprey depending upon the superficial similarities in the respiratory apparatus. But the recent workers on this line do not recognise such an unusual line of phylogenetic relationship.
Relationship with Branchiostoma:
The cyclostomes show many anatomical resemblances with Branchiostoma.
The adult cyclostomes and Branchiostoma possess the following similarities:
a. The notochord is continuous.
b. Presence of segmental muscle blocks (myotomes) from the anterior to the posterior end of the body.
c. There are large number of gill-slits in Branchiostoma and in Eptatretus.
d. Absence of gonoducts.
e. Straight alimentary canal. In addition to the above resemblances, the ammocoetes larva of lamprey is strikingly similar to Branchiostoma. The adult cyclostomes show some anatomical advancement over Branchiostoma but the relationship with the ammocoetes larva needs consideration.
The presence of continuous median dorsal fin, oral hood, endostyle and ciliated gut in both the forms led many zoologists to postulate the idea that Branchiostoma, as seen today, represents a permanent neotenic larval form of some species of cyclostomes. But direct evidences in favour of this contention are lacking.
Remarks:
The relationship with Branchiostoma does not advance our idea on the phylogenetic status of the cyclostomes, but their primitive organisation is amply clear from such a relationship. The cyclostomes possess many advanced characteristics over Branchiostoma and demand a higher rank than Branchiostoma.
Presence of similarities between the cyclostomes and Branchiostoma is possibly due to remote phylogenetic relationship between the Acrania and the Craniates in the evolutionary history of the chordates.
Relationship with Craniata:
Inclusion of the cyclostomes under the subphylum Craniata is unanimously accepted, because all the diagnostic craniate features are evident in them.
These features are:
a. Presence of skull housing the brain.
b. Presence of vertebral column in a lowly developed condition.
c. The epidermis is multilayered.
d. Well-developed circulatory system with a contractile heart and blood is present.
e. Hepatic portal system is present.
f. The kidneys are usually of mesonephric types.
g. Thickening of the myotomal muscles on the dorsal side of the body.
Despite their construction on the basic craniate plan, the cyclostomes differ widely from the rest of the craniates by having numerous individual peculiarities:
a. Absence of biting jaws.
b. Existence of the un-constricted notochord.
c. Rudimentary vertebrae are present.
d. The paired appendages are lacking.
e. Lack of true fin-rays.
f. Possession of an unpaired olfactory organ in adults with two external nasal apertures. The presence of ‘tongue’, sucking mouth, horny teeth, sac-like gill-pouches with supporting branchial basket, dorsal nostril and the ‘thread cells’ in the hagfishes are possibly the adaptive features for their peculiar ways of life.
Relationship with Ostracoderms:
The cyclostomes resemble closely the ostracoderms.
The ostracoderm features present in the cyclostomes are:
a. Absence of complex biting jaws.
b. Structure of brain and cranial nerves is similar.
c. Similar auditory capsule.
d. The pouch-like branchial sacs.
e. Secondarily unpaired olfactory organs in adults. But the lack of fossil records stands on the way of drawing any phylogenetic relationship between the groups. The cyclostomes differ from the ostracoderms, specially by the complete absence of bone and paired appendages and by having a few specialised features connected with the feeding habit.
Despite minor differences, the cyclostomes are regarded to be closely related to the ostracoderms. The specialised features are possibly emerged out of peculiar modes of feeding. Stensio (1927) has advocated the view that the cyclostomes have descended from the ostracoderms and during the process of descent the bony elements have been replaced by cartilages.
Although positive evidences are lacking in favour of this idea, the ostracoderm relationship of the cyclostomes has solid basis to exclude all denials.
Relationship with Fishes:
Although the cyclostomes and fishes are both primary aquatic vertebrates, the cyclostomes present many characteristics which seem to be apparently primitive than the organisation of fishes.
The following features are regarded to be primitive in cyclostomes than in the fishes:
a. Presence of imperfect cranium.
b. Presence of poorly developed vertebrae.
c. Existence of a continuous median fin in Myxine and Ammocoetes larva.
d. The tail fin is of diphycercal type.
e. The heart is a tubular S-shaped structure.
f. The brain is lowly developed.
g. The ninth and tenth cranial nerves are not enclosed in the skull.
h. The nerves are non-medullated.
i. Poorly developed lateral line sense organ.
j. Persistent pronephric duct in hagfishes.
k. The respiratory organs are the gill- pouches.
Remark:
The agnathous vertebrates are phylogenetically more primitive than the fishes (gnathostomes) is not accepted by Stensio (1968) and Jarvik (1968). The cyclostomes have paired olfactory organs and paired external nasal apertures, the features present in fishes. The fishes are phylogenetically more primitive than the cyclostomes.
Phylogenetic Status of the Cyclostomes:
The cyclostomes represent primitive living craniates. They are regarded by many workers as lower in structural organisation than fishes but higher than that of Branchiostoma. But this concept is modified to a considerable extent. The cyclostomes were regarded to be primitive than the gnathostomes (Fishes) but the work on the phylogeny of cyclostomes of Erik Stensio (1968) and Erik Jarvik (1968) establishes the fact that the gnathostomes are more primitive than the cyclostomes.
This aspect is discussed in detail in the last part of this topic. The Cyclostomata is represented at present by the lampreys and hagfishes. The relative status of the two groups — lampreys and hagfishes —within Cyclostomata becomes difficult to interpret.
The lampreys possess many advanced characteristics than hagfishes, viz.:
a. The median fin is divided into two parts, but in hagfishes the fin is continuous.
b. The nostrils are mid-dorsal in position, while in hagfishes they are terminal.
c. Cranium in lampreys is well-developed than the hagfishes.
d. Two semicircular canals are present in lampreys, but only one is present in hagfishes.
e. The vertebral elements are lacking in hagfishes, although these elements are present in lampreys in a lowly developed condition.
f. The hagfishes have functional pronephros which is absent in adult lampreys.
g. The lateral line organs are highly developed in lampreys.
h. The paired eyes are functional in lampreys which are non-functional in hagfishes.
Besides these features, there are a few characters that are definitely more primitive in lampreys than in hagfishes.
These characteristics are:
a. Ill-developed sucking apparatus.
b. The dorsal and ventral roots of the spinal nerves remain separate.
c. The eggs are minute.
d. The cleavage is of holoblastic type.
e. The development is indirect with the Ammocoetes larval stage. This larval stage shows many resemblances with the Branchiostoma.
So the real status becomes more complicated to interprete. With all probabilities, the diversities in the two subclasses of the cyclostomatous are due to their subsequent divergence from a common ancestral source.