Zool 4310/5310
The purpose of this lab is to become acquainted with the gross external and internal anatomy of the perch. This “lab manual” has been provided for you to work through, and several prepared models are available for study. By the end of the lab, be sure that you can identify and describe the structures indicated in boldface type.
The perch belongs to the Osteichthyes, which is comprised of fishes in which the skeleton is partly or largely bony, as opposed to the cartilaginous skeleton of chondrichthyans. Most living bony fishes are in the Teleostei, the group to which the perch belongs (order Perciformes, family
Percidae). It is a common freshwater fish in some parts of the country.
External Anatomy
The body of the perch is laterally compressed, in contrast to the depressed body of the dogfish. Despite this difference, however, in the perch, as in the dogfish, the widest part of the body is about one-third of the distance from the head to the tail. As indicated previously, this streamlined shape allows fish to swim through the water with a minimum of resistance.
The body of the fish is divided into head, trunk, and tail without clear demarcation of the regions. Several fins are present. On the dorsal surface locate the anterior dorsal fin and the posterior dorsal fin. Unbranched spines support the anterior dorsal fin, while softer branched fin rays occur in the posterior dorsal fin. The caudal fin, or tail, differs in shape from that of the dogfish in that the dorsal and ventral portions of the tail in the perch are essentially the same size and shape. This is a homocercal type of tail, as opposed to the heterocercal tail of the dogfish.
On the ventral surface in the midline note the single anal fin, a structure that is not present in the dogfish. Still farther anteriorly are the paired pelvic fins, which are sometimes called the ventral fins because of their forward position, compared with the primitive position, which is farther posterior. In many teleosts the pelvic fins have migrated anteriorly, and in a few species they may even be anterior to the pectoral fins themselves. On the sides, just anterior to the pelvic fins, are the pectoral fins, which still retain their primitive positions. The principal support for the fins (other than the anterior dorsal fin) are the soft branched fin rays, although a few of the fins sometimes have a spine or so on the anterior margin. A lateral line occurs along the body, one on each side.
The scales differ considerably from those of the dogfish. Take a probe or scalpel and remove several. Note that they are rounded and that they overlap when on the fish. They do not have the small point near the center, as does the placoid scale. The placoid scales found in the
1

2 dogfish are considered to be homologous to the true teeth of higher vertebrates. They are derived embryologically from both ectoderm and mesoderm and exhibit the essential structure of teeth. In contrast, most of the scales found in the perch are ctenoid scales. Scales of this type are wholly mesodermal in origin and therefore not homologous to teeth. The buried edge of the ctenoid scale is somewhat toothed; from this fact the name ctenoid (comblike) is derived. The toothed edges serve to anchor the scales more firmly in the dermis .
Turn next to the head. The mouth is terminal , in contrast to the ventral mouth of the dogfish. Just anterior to each eye is a pair of external nares, or nostrils. The most posterior opening of each pair is rather large and is slightly dorsal to a line drawn across the center of the eye. The posterior openings are not covered. The anterior opening is about 3 mm ( 1/8 inch) anterior and is covered by a protecting flap; it is some what smaller than the posterior opening.
The two pairs of nares probably result from a secondary division, since there is only a single nasal capsule for each pair of nares on each side. Probe into one of the openings and note that the nares do not connect with the mouth cavity. In most air-breathing vertebrates a connection with the outside is established from the mouth or pharynx by way of the external nares. In these groups, the nares function in gaseous exchange, but in fish they perform only an olfactory function.
On each side of the head is a flap-like structure, the operculum, which covers the gill slits and the gills. All gill slits thus open to the outside by a single opening between the operculum and the body wall, rather than individually like those of the dogfish. Lift the operculum and observe the gills and gill slits that are underneath. As compared with the dogfish, the number of gills and gill slits has been reduced; only four of each are present. Recall that, in the dogfish, the gill slits were widely separated by the interbranchial septa. These septa are practically nonexistent in the perch; only the gills and associated structures are present between the slits.
The gills are supported by bony gill arches, on the edges of which are fingerlike gill rakers. On the posterior edge of the operculum is a complicated structure composed of a membrane supported by bony rays. The membrane is the branchiostegal membrane, and the bony supports are the branchiostegal rays. This structure aids in drawing the water through the mouth and the pharynx and out through the gill slits.
Just anterior to the anal fin is a comparatively large opening, the anus, and just posterior to the anus is a much smaller opening, the urogenital pore, which is sometimes difficult to locate in preserved specimens. Since the digestive and urogenital systems do not open into a common chamber, there is no cloaca in the perch.
Internal Anatomy
The internal organs can be best studied by removing one side of the body wall. Make a midventral incision from the anus anteriorly to about the level of the pectoral fins. Then extend the cut dorsally to just above the most posterior projection of the operculum. Make a similar lateral and dorsal cut from the region of the anus to about the same level dorsally. Now lift this flap of tissue and entirely remove it from the body. In some specimens a large quantity of white and yellow fatty tissue will cover the internal organs; this should be carefully removed with forceps.
Dorsal to the visceral organs is the gas bladder, or swim bladder, a large membranous structure that comprises the upper (dorsal) half the abdominal cavity. This structure is not present in the

3 dogfish. Since the walls of the swim bladder are relatively thin, they may have been broken when the side of the body was removed, in which case there will be a large empty space in place of the membranous swim bladder. The principal function of this structure in the perch is hydrostatic , since in the adult there is no connection between it and the pharynx (the physoclistous condition).
The visceral organs are contained within the pleuroperitoneal cavity, or abdominal cavity , which is the posterior division of the coelom. The anterior portion of the coelom, the pericardial cavity , contains the two-chambered heart and associated structures. Although you should dissect away enough tissue to uncover and examine the heart, we will not study it in detail in the perch because it is quite similar to that of the dogfish. The walls of the pleuroperitoneal cavity are lined with peritoneum, and mesenteries help to hold the organs in place.
Digestive System. Locate the liver, the large organ at the anterior end of the abdominal cavity.
In life this structure is reddish, but in preserved specimens it is sometimes gray or whitish. The liver is somewhat lobed. A gallbladder and a bile duct are also present but are sometimes hard to locate. The bile duct opens into the duodenum, as in the dogfish.
With forceps, pull away most of the liver to expose the organs underneath. Locate the esophagus, the very short tube covered by the liver. It is difficult to distinguish the line of demarcation between esophagus and stomach.
The stomach is divided into two definite regions, a cardiac portion, which is a direct continuation of the esophagus and which posteriorly forms a blind pouch, and a smaller pyloric division, which protrudes somewhat ventrally from the cardiac region.
Between the pyloric portion of the stomach and the anterior end of the intestine
( duodenum ) is the pyloric sphincter, which forms the dividing line between the stomach and the intestine. The pyloric sphincter is somewhat difficult to identify in the perch, however, since in this region there occur three fingerlike blind pouches that open into the duodenum. These are the pyloric caeca (singular, caecum, = cecum ), the function of which is to increase the digestive and absorptive surfaces of the digestive tract. From the pyloric caeca the intestine continues anteriorly for a short distance and then turns posteriorly. Two or three loops are usually made before the intestine opens to the outside at the anus. The coils of the intestine may be seen after the stomach has been raised.
The spleen is the small, rounded, usually reddish body within the coils of the intestine. It is not a part of the digestive system but probably functions in connection with the circulatory system. There is no definite pancreas, but scattered groups of cells have been discovered in the walls of the intestine that probably correspond to the pancreas of other vertebrates.
The Urogenital System. Break the wall of the gas bladder if it has not been previously broken.
You should study both male and female systems. The kidneys and mesonephric ducts
(Wolffian ducts) are similar in the two sexes. The kidneys are not very definite structures but greatly resemble those found in the dogfish. They are of the mesonephric type. These structures occur as two elongate bodies on the dorsal surface of the body wall above the swim bladder, one

4 on each side of the vertebral column. The mesonephric ducts are quite small and rather difficult to find. They emerge from the posterior ends of the kidneys, combine into a single duct, and enlarge to form a small urinary bladder just before emptying to the exterior by was of the urogenital pore.
Male Reproductive Organs.
The reproductive organs, or gonads, vary considerably in size during the year; if the fish was captured during the reproductive season, the gonads may be quite large. Locate the testes, a pair of ribbonlike structures (unless enlarged for reproduction) just above the intestine. From the posterior end of each testis, a tube, called the vas deferens, leads to the urogenital pore and thence to the outside. In most other vertebrate animals with a mesonephros (plural, mesonephroi), the mesonephric duct performs the functions of both excretion and reproduction in the male. It thus seems probable that these extra tubes, the vasa deferentia, are not homologous with the vasa deferentia of other vertebrates that have mesonephroi.
Female Reproductive Organs.
There is a single ovary in the female, which results from a fusion of two ovaries in the embryo. As in the male, the reproductive organ will vary in size with the season of collection. The ovary is usually more granular in appearance than the testes.
There are no oviducts or Mullerian ducts as such. The tube that extends posteriorly from the ovary is attached directly to it and is simply a posterior extension of the ovary itself; it is therefore not considered to be homologous with the oviduct of other vertebrates and will be called the ovarian duct. In most other vertebrate animals the oviducts and the ovaries are not directly connected. The ovarian duct passes posteriorly and opens to the outside by way of the urogenital pore.
The circulatory system, sense organs, nervous system, and other structures are very similar to those found in the dogfish.

5

6