Dissected so as to show its nervous, respiratory, circulatory, digestive, and reproductive systems in situ.
The following external characters are to be noted: - the laterally flattened body and pointed head: the general investment of cycloid scales: the conformation of the mouth: the large eye devoid of eyelids; and the two dorsally placed apertures, anterior and posterior, of the nose, neither of them communicating with the mouth: the lateral line of sensory organs extending down each side of the body to the tail, and the four depressions on the under surface of each lower jaw indicating the position of the sense organs contained within one of the so-called mucous canals of the head: the opercular apparatus, composed of the operculum and the branchiostegal membrane with its supporting rays which cover the branchial arches laterally and meet on the ventral surface under the chin: and the two sets of fins, azygos and paired. The former consist of the two dorsal, the caudal and the anal fins, imperfectly seen in this specimen, because the caudal region has been removed and the body cut through about the middle of the second dorsal and the anal fins. The paired fins are to be seen on the left side - the pectoral above, i.e. dorsally, and the ventral below.
A line drawn through the attachment of the pectoral fin, and at right angles to the long axis of the body, passes just in front of the ventral fin; the latter is therefore said to be thoracic in position. When such a line passes behind the ventral fin, the latter is said to be jugular in position. In a certain group of Teleostei which preserves the duct to the air-bladder and is hence termed Physostomi, the ventral fins are placed somewhat in front of the anus, but remote from the pectoral, and are then said to be abdominal, e.g. Pike, Salmon.
The brain and viscera have been exposed in situ by the removal of the roof of the cranium, the right opercular apparatus and right side of the body walls.
There are four divisions of the brain visible. They do not overlap one the other, nor do they fill the cranial cavity as in the young frog. The first division forms the olfactory lobes or rhinencephala from which the olfactory nerves may be seen passing forward: the second division the cerebral hemispheres or prosencephala: the third the optic lobes, corpora bigemina or mesencephalon, the largest of all the divisions in Teleostei: and the fourth the cerebellum which is subglobular and, unlike the preceding parts of the brain, unpaired.
The removal of the operculum on the right side displays the four gill-arches with their double series of gill-filaments, arranged like the teeth of a comb. Each arch is hence said to be bi-pectinate. Internally to the last or fourth gill-arch, and anteriorly to the liver from which in the natural state it is separated by a fibrous septum, lie the heart and ventral aorta. The aorta has a distinct bulb or swelling where it springs from the ventricle: this, the most muscular part of the heart, is in contact with the body walls ventrally while the thin auricle is placed dorsally to it. The liver is large and imperfectly divided into three lobes of which the left is not exposed to view. The single ovary lies posteriorly to the liver and being ripe occupies the greater portion of the abdominal cavity, having displaced the remaining viscera. The anterior three-quarters of its right side have been removed, but owing to the extreme state of distension of the organ, it is not possible to make out the transverse ovigerous lamellae which cross its interior. The commencement of the duodenum with one - the shortest of the three pyloric appendages, may be seen between the liver and ovary.
The two other appendages, the stomach, and the loop of intestine containing the spleen in its concavity, are all alike hidden on the left side of the body. The gall-bladder has been displaced upwards and to the right. It is lying on the under surface of the liver in a depression, the homologue of the fossa cystis felleae of man, with which it does not in this fish usually come into relation save when the ovary is in a state of turgescence.
The terminal portion of the intestine and the rectum pass with a straight course down the middle line of the body to the anus. This aperture, into which a black bristle is inserted, is superficial in Cyclostomi, in Teleostei and Ganoidei, and it is placed in front of the genito-urinary depression, clearly visible here behind it. The air-bladder lies dorsally to the ovary, and between its upper end and the liver posteriorly, and the fourth gill arch anteriorly is a gland (really paired) the homologue of the thymus. Bands of yellow-coloured fat in a state of atrophy correlated with the hypertrophy of the ovary, are to be seen contained in the peritoneal lamellae which unite the intestine to the ventral surface of the ovary as well as along the outer or attached edge of the air-bladder.
An Acanthocephalous parasite, Echinorhynchus Proteus, is not uncommon in the intestines. E. angustatus is also found in the same place as well as the Nematode Cucidlamis elegans and the remarkable Cestode Triaenophorus nodulosus. A variety of other parasitic worms may be met with, a list of which is given in Zschokke's work cited below (p. 90) or in von Linstow, Compendium der Helminthologie, Hannover, 1878, p. 206, an invaluable work for all students in this difficult branch of Zoology.
The epidermis consists of several layers of cells, which are united by protoplasmic processes. The external cells generally bear a striated cuticle. Glandular cells are present and open on the surface. There are also 'Retort-cells,' or 'Kolben-zellen,' with clear contents, which rise to the surface and burst. The scales are dermal ossifications, and doubly refractile. They are covered by the epidermis, and contained within pouches of the dermis. When the free border is evenly, or nearly evenly round as in the Perch, they are said to be cycloid; when it is produced into more or less prominent teeth, they are said to be ctenoid, as in many other Acanthopteri. The scales of the lateral line are modified. They are perforated for the passage of a nerve, and channelled on the outer surface near the free edge. The channel towards the base of the scale is converted into a canal for the protection of the sense-organs of the lateral line.