Animal Systems Information II

Phylum Echinodermata

Biology 125 Biology for Science Majors II Lab Manual. Written by Dr. Tristyn Hay October, 2021. Some content provided by the University of British Columbia and Okanagan Biology Graduate Program students handbook The Fictional Animal Project: A Tool for Helping Students Integrate Body Systems. Adv. Physiol. Edu 41: 239-243 Blatch et al. 2017

A “spiny skin” characterizes members of the phylum Echinodermata, by only living in marine environments, and by possessing a unique water vascular system. The are also radially symmetrical as adults, but not as larvae. The spiny skin is actually an endoskeleton made up of calcareous plates. Typical echinoderms include seastars (starfish), brittle stars, sea urchins, sand dollars, sea cucumbers and sea lilies. According to DNA evidence, they are closely related to members of the Phylum Chordata, the phylum to which humans belong. Chordates andechinoderms also share a similar pattern of embryonic development, as both are deuterostomes.

EXTERNAL FEATURES OF THE SEASTAR

Pisaster can be seen in the crevices of intertidal rocks and covering the pilings of piers and docks. Pisaster is a rapacious predator on bivalves, but little eats it, except for the occasional desperate seagull.

The underside of the seastar is referred to as the oral side; the opposite side is the aboral side. The external features on the aboral side include the rays (arms) and sieve plate (madreporite). On the surface there are small calcareous bumps called spines and between them arestructures called dermal papillae (not visible with the naked eye). On the mouth side tube feet, the mouth and ambulacral groove can be observed.

The sea star uses its water vascular system to capture food and in locomotion and respiration. Water enters the system via the sieve plate. The water is then moved along through a series of canals by the action of cilia out to the rays and to the ampullae. Contraction of an ampulla forces water into a tube foot lengthening it. Contraction of longitudinal muscles in the foot forces water back into the ampulla shortening the foot. Small suction discs at the end of the feet allow the starfish to move on hard ground and to grasp its prey. The tube feet are also important sites of gas exchange. Pressure generated by the water vascular system also allows the seastar to evert its cardiac stomach and pry open the shells of bivalves like clams, in order to eat them. It is important to realize that the water vascular system is not a true a vascular system supplied with a heart as a pump, and that there is no blood that contains special pigments for transporting gases (as is seen in other animals with true vascular systems). Video of tube feet in action

DIGESTION

Seastars feed by everting part of the cardiac stomach through their mouths out into the water (or even directly into the shells of molluscs). Then the food is either brought back into the body of the seastar for digestion or digested in place outside the seastar’s body. This allows it to feed on animals much larger than itself. Without being able to evert one of its stomachs, the hard endoskeleton would limit the size of prey that could be eaten by the seastar. The cardiac stomach passes the food to the pyloric stomach, where more digestion takes place. Most of the space in the ray is taken up by two highly branched digestive glands (also called pyloric caeca). They secrete digestive juices from their many lobes. They are connected to the pyloric stomach by a pyloric duct in each ray. Absorption of nutrients occurs in the digestive glands.

TRANSPORT AND EXCHANGE

Gas exchange occurs through two types of structures that extend from the surface of the seastar: the tube feet and the papillae (also called papulae or gills); the latter are thin-walled extensions of the body cavity (coelom) that extend onto the aboral surface of the seastar. They look like small finger-like sacs when the animal is in the water, but may not be visible on these specimens. There is no circulatory system in the seastar. Instead, gases are carried around the body via fluids in the body cavity, powered by the action of cilia.

In one ray the digestive glands have been removed to show the ampullae of the tube feet, which are the bulbs on the aboral side of the tube feet, which contract to extend the tube feet.

EXCRETION AND OSMOREGULATION

In seastars, nitrogenous waste is excreted as ammonia. Solid waste is expelled through the anus. There is no osmoregulation in seastars. They are osmoconformers.

REPRODUCTION

Sometimes individual animals are male or female (dioecious). In other cases, the same gonadcan produce both eggs and sperm, either simultaneously or sequentially. External fertilization occurs after the gametes have been released through ducts located on the central body between the arms.

Phylum Chordata-Perch

Phylum Chordata includes all those animals (perhaps 50,000 species) with backbones: fish, amphibians, mammals, reptiles, birds, as well as a few other species. A few chordates do not have backbones and are thus classified as invertebrates. These include small marine animals called tunicates and lancelets. To be a chordate an animal must:

1. Have had at least during embryonic development, a structure called a notochord. (this is a flexible supportive rod running longitudinally through the dorsum of the animal just ventral to the nerve cord; it becomes the spinal column in vertebrates);
   
2. have pharyngeal gill slits at some stage in their development;
3. have a dorsal hollow nerve cord, and
4. Have a post-anal tail.

The perch is a chordate belonging to the Actinopterygii (ray-finned fishes) within the subphylum Vertebrata. Well over 25,000 ray-finned fish species are known today and, as we probe deep ocean regions, we will find many more. Perch are freshwater food and game fish found in Europe and North America. Perca fluviatilis or yellow perch is a marginal sport fish, which has been introduced into Vaseux and Osoyoos Lakes in the Okanagan.

THE PERCH – EXTERNAL FEATURES

Be sure that you can identify the dorsal and ventral sides of your specimen and its anterior and posterior ends. Be able to draw and identify the pectoral, dorsal, pelvic, anal, and caudal fins.
Most fish have external fertilization, but a few species, like guppies, have internal fertilization and are livebearers.

DIGESTION

Perch are carnivores. Their digestive tract comprises an alimentary canal (the tube running from the mouth to the anus) as well as accessory structures, such as the liver, salivary glands and pancreas. After the mouth, the alimentary canal includes, in order, the esophagus, stomach, pyloric caeca (secretory and digestive functions), intestine and duodenum.

CIRCULATION AND RESPIRATION

Perch, being aquatic chordates, use gills as a respiratory exchange surface. Cut away the bony operculum to expose the gill chamber and its four gills. Remove an individual gill to study its structure. The hard-bony support is the gill arch, with posteriorly directed gill filaments. The hard-anterior finger-like projections, the gill rakers, prevent coarse material or food from passing through the gill slits. Gill filaments are supplied with capillary beds across which gas exchange occurs. The flow of blood is opposite to the flow of water across the gills, called counter-current flow; this enhances the concentration gradient and thus the rate of diffusion. This helps metabolically active fish to more efficiently extract oxygen from relatively poorly (relative to air) oxygenated water.

Chordates have closed circulatory systems. The advantage of a closed circulatory system over an open system is that blood can be kept at a higher pressure thus moving through the body more quickly, which is more efficient at servicing body tissues.

The pericardial cavity, just beneath and ventral to the gills, contains the two-chambered heart. Observe the thin-walled posterior atrium extending over the thicker-walled ventricle. Anterior to the ventricle is the bulbus arteriosus, the enlarged base of the ventral aorta that carries blood to the gills and on to the rest of the body. The blood is under pressure when it reaches the gills, but pressure is reduced when the blood passes through the capillary bed. Blood pressure remains low as the blood travels to the rest of the body. This is different from animals with a pulmonary loop, in which the blood travels back from the lungs to the heart, where it is pumped (re-pressurized) to the rest of the body and thus can move more quickly and service body tissues more efficiently than the lower pressure system.

EXCRETION AND OSMOREGULATION

Chordates have kidneys composed of nephrons. In fish and amphibians, the kidneys lie as two straps alongside the spinal column; in reptiles they become attenuated to the posterior portion of the animal, and this phenomenon is even more advance in birds and mammals. Kidneys function as both excretory and osmoregulatory organs. In addition, osmoregulation can be facilitated by other organs, depending upon the type of animal.

In freshwater fish like the perch, the kidney excretes large amounts of dilute urine in order to rid the body of excess water. In marine fish, the kidney excretes salt ions in very concentrated urine. In all fish, ammonium is the form of nitrogenous waste excreted.

Phylum Chordata-Rat

Rats are examples of terrestrial chordates. They are also mammals. Therefore, their anatomy will reflect the fact that they get their oxygen from air, must conserve water, and have internal fertilization with their offspring then developing in a uterus before being born.

DIGESTION

As in all tetrapods, the anterior end of the digestive and respiratory systems is shared. Air, water and food all pass through the oral cavity. In the pharynx, the epiglottis acts as a gate to direct air into the lungs and food or water into the rest of alimentary canal. Like in the perch, the digestive system comprises the alimentary canal (oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus). Each part of the alimentary canal is specialized for a particular function: mechanical or chemical digestion, absorption of water or nutrients, or retention or elimination of undigested solid wastes. Be able to identify the stomach, small intestine, salivary glands, liver, and spleen in preparation for your lab exam.

The accessory structures of the digestive tract include the liver, pancreas and salivary glands. These are exocrine glands, meaning that they secrete chemicals into ducts that lead directly to another structure - in this case, the alimentary canal specifically, the accessory structures supply enzymes, or other chemicals that aid in digestion, to the alimentary canal.

CIRCULATION AND REGULATION

After being directed from the pharynx into the respiratory system, air enters the trachea, the bronchi and then the lungs. The lungs of amphibians are fairly simple invaginated sacs of vascularized epithelial tissue in which there are internal partitions to increase the surface area. Birds and mammals, however, have lungs of a more complex nature. In mammals, the bronchioles divide into finer and finer branches until they end in tiny, blind-ended sacs called alveoli, which greatly increase the surface area across which gas exchange occurs. Birds have other mechanisms to increase gas exchange.

Because air movement into and out of the lungs is based on differences in air pressure, it is important that the lungs be sealed in an air-tight part of the body where movement of muscles can create differences in pressure. This area is called the thoracic cavity. Differences in pressure within the thoracic cavity are created by movement of muscles associated with the ribs, as well as the diaphragm.

The heart of the rat, like all mammals, consists of four chambers: two atria and two ventricles. When the atria contract they fill their respective ventricle with blood; when the ventricles contract, the increased pressure directs blood from the right ventricle to the lungs and from the left ventricle to the rest of the body. Your rat has been double-injected with latex. Blue latex was injected into the veins and red latex was injected into the arteries. Entering the right atrium are three main blood vessels that bring the deoxygenated blood back to the heart from all regions of the body. These blood vessels are the right superior vena cava, the left superior vena cava and the inferior vena cava. The right and left superior venae cavae return deoxygenated blood to the heart from the right and left side of the head, neck and forelimbs. The inferior vena cava returns deoxygenated blood to the heart from the lower part of the body. More in depth detail of a mammal heart will be covered in the next lab.

EXCRETION AND OSMOREGULATION

Rat kidneys, like those of humans, excrete urea. Urea is less toxic than ammonia therefore it requires less water to be excreted; however, energy must be expended in order to make urea from ammonia. Organisms that use urea tend to live in environments where water conservation is important. Rat kidneys lie outside of the abdominal cavity, behind the lining of connective tissue. Thus, they are said to be retroperitoneal. The kidneys are the major osmoregulatory organ in the rat.

REPRODUCTION

In male rats, the scrotum, a large sac of skin, muscle and connective tissue holds the testes on the exterior of the body just ventral to the anus. During non-breeding periods, the testes may be retracted into the abdominal cavity and the scrotum will not be enlarged. Around the outside of each testis is a C-shaped structure known as the epididymis, which is a very long, highly coiled tubule. Part of the epididymis is on the posterior end of the testes and part is on the anterior end. Sperm cells produced in the seminiferous tubules of the testes pass into the epididymis and then into the vas deferens, which is a moderately large tube leading from the epididymis to the urethra, which is a tube located inside the penis. The penis is enclosed in an epithelial sheath and held along the ventral wall of the abdomen.

The prostate glands are found on either side of the urinary bladder. These glands, and some other glands in this region, comprise the accessory sex glands. The secretions of these glands form the seminal fluid, which carries the sperm during ejaculation, activates and provides certain nutrients for them, and contains substances that neutralize the somewhat acid environment in the vagina.

In female rats, as in all mammals, the eggs are produced in the ovaries and then released into the oviducts, through which they travel to the uterus. If fertilization occurs, the fetuses develop in the uterus. The rat uterus has two horns, which join at their base to form the vagina.