Herrick (1901) described three types of lateral line organs in the catfish, Ameiurus nebulosus: canal organs, small and large pit organs. Their structures and distributions in teleosts of Japan have been described by Sato (1954, 1955a, b, 1956, 1963).
The lateral line system of the generalized teleost is characterized by canal organs in the head and mid-lateral region of the flank and pit organs on the whole body. Neuromasts of canal organs are present in tunnel-like canals embedded in the dermis. The canals open at intervals to the exterior. Pit organs are present in pits. Absence of the lateral line scales with sensory canals in the flank is one of characteristic features of killifishes. It is true of the medaka, also. In searching for the other sense organs belonging to the lateral line system, four pairs of collateral rows of 'superficial sense organs' with structures essentially similar to the 'small pit organs' in most teleostean fishes have been found on the trunk and tail. Though the superficial sense organs are projected slightly over the epidermis and not sunk in pits, they are called here pit organs for the sake of convenience. In the head, there are large sense organs in grooves besides the small pit organs. Although a paper pertinent to this subject was read before the Annual Meeting of the Zoological Society of Japan held in 1947 and its abstrast was published (Yamamoto, 1947), the full account with illustrations has not been given until now.
The small pit organs cannot be seen in intact living fish because they are translucent. They can be barely observed in formalin-fixed specimens. Under a low powered microscope, they are seen narrowly as a group of tiny circular bodies (34-40 micron in diameter) only when the angle between the falling light and the line of vision is proper. Small pit organs illustrated in Figures 10-1 and 10-2 are results of observations by this method.
A survey of literature revealed that Denny (1937) observed 'superficial organs' by immersing living Fundulus heteroclitus in a dilute solution of methylene blue for 15 to 20 hours. In the medaka, immersion of living fish in 0.002 percent solution of this dye for two days is recommended. The dye is taken up not only by granules of the sense cells in the neuromasts but also mucous cells surrounding them. So that this method may be used only for approximate location of the small pit organs.
In the trunk and tail, four pairs of rows of small pit organs are present; (1) dorsal, (2) dorso-lateral, (3) ventro-lateral, and (4) ventral (Fig. l0-2). Of these, the dorsal pair originats at the posterior head region and gradually unites each other when they reach the mid-trunk level. The ventral pair ends before the anus. It may be remarked that no neuromasts are found on the mid-lateral region of the flank where typical pored lateral line scales are present in most fishes. The situation in the medaka is quite different from that of Fundulus in which the superficial sense organ system corresponding to small pit organs consists of a main row of each mid-lateral region and two shorter lines dorsal to the main (Denny, 1937).
A group of the small pit organ consists of three to five neuromasts on the skin (Fig. 10-1). Each neuromast projects slightly over the surface instead of lying in a pit. (Fig. l0-3) There are two types of cells making up the organ: sensory cells and sustentacular ones. The sensory cell is a pear-shaped one tapering towards its distal end and located in the superficial half of the organ. Its nucleus is large and centrally located. There are granules in the cytoplasm distal to the nucleus. It bears sensory hairs on its outer surface. The sustentacular cell is an elongated cell which extends through whole depth of the epithelium, with a nucleus at the proximal end. At the free surface of the small pit organs, tiny terminal bars are present between the sensory and sustentacular cells. Each pit organ is innerved by a collateral nerve.
It may be remarked that the sensory cells of the taste organs of teleosts extends through whole depth of the epidermis by which it can be distinguished from those of the pit organs which does not extend the whole depth of the epidermis.
In the head region, large neuromasts in groves are present besides small pit organs (Fig. 10-4). These large sense organs seem to correspond to the 'head canal organs'. Five series of the large groove organs can be seen: (1) supraorbital, (2) occipital, (3) preorbital, (4) infraorbital, and (5) postorbital besides many pit organs. The small pit organ system of the head consists of four rows: (1) rostral (supra-maxillary), (2) mandibular, (3) periorbital, and (4) opercular. The structure of the head pit organs is essentially similar to those in the trunk.
Each neuromast in the groove organs bears a large transparent jelly-like cupula (170 micron height) on its surface (Fig. 10-4). The cupula cannot be seen in intact living fish because it is transparent. Presence of the cupula can be demonstrated by immersing living fish in 0.004 percent solution of Wasserblau (Grubler) for two days. The surface of the cupula is stained blue by this dye. The cupula is stiff and flexible. The structure is homologous with the crista ampullaris of the inner ear. Gerard (1936) inferred that the inner ear of vertebrates has phylogenetically derived from a primitive lateral line system. Structure of the large pit organs is essentially similar to that of the small ones, viz., it consists of sense and sustentacular cells. Presence of the cupula on the small pit organ is difficult to demonstrate in living fish treated with Wasserblau probably because of its smallness. In histological sections, remnants of the cupula can be seen on the summit of the small pit organs.
Embryologically lateral line organs arise from the skin in common with the internal ear and anatomically their innerations are in common with those of the internal ear. The nerve fibers from the entire lateral line system and the internal ear converge into a common acoustico-lateral area within the brain (Herrick, 1926).
Detailed researches on the innervation of lateral line organs have been carried out by Manigk (1934) in Phoxinus laevis, Dykgraaf (1934) in Nemachilus barbatulus, Denny (1937) in Fundulus heteroclitus, Stensio (1947) in fishes and amphibians, and Jakulowski (1963) in Acerina cernua. Baily (1937) performed an experimental study on the origin of the lateral line organs and Woellwarth (1934) carried out experiments on the relationship between these organs and the lateral nerves.
These researches confirmed that the lateral line organs, whether located in canals or superficial in position, are supplied by fibers of lateral nerves originated from the area of acoustico-lateralis of the medulla oblongata. In the trunk, these organs are innervated by the posterior fibers in the X cranial nerve (vagus) while those in the head region by the anterior fibers in the complex of the VII (facialis) plus the V (trigeminus), both of which contain other nerve fibers in addition. Innervation of the lateral line system of the medaka has not been worked out.
The function of the lateral line organs in aquatic vertebrates has been a subject of interest to biologists. Because we have not such sense organs, various conflicting notions have been proposed as to their possible function. Parker (1904) reviewed literature concerning their function and presented the results of his experiments systematically carried out on Fundulus heteroclitus and confirmed with other fishes. Parker concluded that these organs are mechano-receptors responding to low vibrations of water (six per second), and that they are not stimulated by light, heat, salinity, food, oxygen, carbon dioxide, foulness of water, water pressure, water currents or sound.
A strong argument against Parker's notion of the lateral organs as functioning as receptors for low vibrations was presented by Hofer (1908). His experiments were carried out systematically on the pike (Esox lucius) and confirmed with other fishes. In order to obtain fish in which lateral line organs are non-functioning, the lateral line nerve was cut at the level of the pectoral girdle and the lateral line organs at the head were cauterized. In addition, the cornea was made turbid by cauterization in order to eliminate possible effect of visual sense.
Response to water vibrations was indistinguishable between normal and operated fish. According to him, water vibrations are adaptive stimulus of general sense of skin but not specific stimuli to lateral line organs.
Hofer showed that the adaptive stimulus to the lateral line organs is water currents toward fish body. Water currents elicit the response of dorsal-fin erection in the normal pike. The response is induced even by water currents which are so weak that they do not elicit labyrinth excitation. Operated fish do not respond to such a weak water currents. They react only to strong currents which displace fish body causing excitation of the labyrinth.
Reception by the lateral-line organs of weak currents renders it possible to avoid collisions with objects which may either be moving toward the fish or towards which the fish moves. Thus, fish can avoid collisions with a wall by reflective distrubance of water from it when fish approaches it, even in the dark.
Hofer inferred that the lateral line organs would also be excitated by strong currents causing fish to orient towards water currents (rheotropism) but excitation in the labyrinth would be predominant in propagatory swimming movements.
It was unfortunate that Hofer's article has appeared in a poorly circulated journal so that his notion has not been known to American authors. His paper was reviewed in details and evaluated by an Italian physiologist Baglioni (1913).
In this connection it may be remarked that de-eyed goldfish and other fishes are insensitive to rectilinear displacement but respond to all curvilinear movements involving rotations about one of the major axes of the body. The response is of a dynamic compensatory type determined by the semicircular canals of the labyrinth (Gray, 1937).
Lyon (1904, 1909) has pointed out that the rheotropism in fish is due to the orienting effect of moving retina images. He showed that fishes orient themselves just as well when they are put into a closed glass bottle which is dragged through the water. Rheotropism has been interpreted as compensatory movements to keep the visual image of the same object fixed. According to Lyon all the phenomena of rheotropism ceased in the dark or when the fish were blind. This statement may come into question. Rheotropism in fishes seems to be held by triple assurance, i.e. the eye, the labyrinth and the lateral-line organs. In the light, the visual sense may be predominating while in the dark the labyrinth is predominating and the lateral-line organs is subordinating. The lateral-line organs seem to be receptive to even to such weak water currents which do not elicit exitation of eye and the labyrinth.
In the medaka, rheotropism is persistent in the dark in less extent. The same is true of blinded fish in which lenses are removed under anesthesia. The response is positive at least in curvilinear displacement of fish. Labyrinth excitation seems to be predominating in the dark and blinded fish. Blinded medaka shows avoiding reactions to water current produced by sudden approach of a glass rod. The fish in which cupulae in the head are removed and nearly all pit organs in the head, trunk and tail are mechanically destroyed under anesthesia, fails to respond to such weak currents after recovery from anesthesia. These experiments support Hofer's conclusion that the adequate stimulus of the lateral line organs is water currents.
It is interesting to recall that the sensory cells of both lateral line organs and the internal ear have the homologous structures, viz. the cupula and the crista ampullaris, respectively. These flexible structures are lying on the sensory hairs of sensory cells. Bending of the cupula by water currents and that of the crista ampullaris by disturbance of the endolymph of the semi-circular canals cause excitation of sensory cells of both organs.
Hoagland (1933a, b) showed that impulses from the lateral line organs of the catfish are in a state of continuous activity. The frequency of the spontaneous discharge is an exponential function of temperature according to the Arrhenius-Crozier equation. The finding, however, would hardly be taken as the evidence that the lateral line organs as thermal receptors since nearly all biological processes, involving heart beat and ciliary movements, are exponential functions of temperature.
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